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In the science fiction series Stargate SG-1, the Replicators are a race of self-replicating machines, and arguably one of the most advanced races in the Stargate universe. In the SG-1 spinoff Stargate Atlantis, a new form of Replicators were introduced that are believed to be the very first replicators created by the Ancients.

They were first mentioned indirectly in the episode "Fair Game", when Thor informs Jack O'Neill about an enemy worse than the Goa'uld. The Replicators themselves first appear in the episode "Nemesis". Throughout the series, the Replicators are primarily the main enemies of the Asgard, but their expansionist nature makes them a significant threat to all other lifeforms in the universe, including the people of Earth.

Standard Replicators are composed of modular blocks, which can be compared to individual computers, that come together to perform several tasks. They have very high interconnectivity, and are able to communicate via subspace to other Replicators who may be light-years away. Every block contains two million isolated kiron pathways, that control the molecular cohesion of all Replicators. These blocks can self-assemble into various larger forms, the most common being the small spider variation illustrated above.  However, much larger forms have been observed, including whole spacecraft on a scale comparable to Goa'uld Ha'tak.  The blocks themselves are composed of whatever materials were available at the time of construction (implying an inability to transmute elements). For example, a Replicator infection on board a Russian submarine resulted in Replicators made of steel, even susceptible to rusting. Most Replicators however are made of more advanced alloys absorbed from Asgard ships, with whom the Replicators have been waging war for some time. It takes multiple blocks to create more blocks, although a single Replicator appears to be capable of self-replication.

Replicators are impervious to all known handheld energy weapons, including Zats and Staff Weapons. Though projectile weapons can shatter standard Replicators, the resistant machines have demonstrated a limited ability to reassemble from the scattered blocks and continue with their objectives.  However, this ability was lost as the show progressed. To defend themselves at close quarters, a Replicator is capable of spraying an assumed acidic liquid, known as Replicator Spray.  This liquid is capable of penetrating all metals, even the nigh-on invincible armor worn by the Kull Warriors.  Replicators are also incredibly strong in relation to their size. A single standard Replicator has been seen capable of holding open the SGC's Gate Room hydraulic blast doors.

The Replicators are presented as being one of the most technologically advanced species in the Stargate universe, with the possible exceptions of the Ancients and the Ori. They not only swiftly assimilate foreign technology, but also enhance it, usually far beyond the understanding of its previous users. For example, Replicators can modify a Goa'uld Ha'tak to travel at 800 times its maximum speed, and utilize an Asgard time-dilation device to accelerate time and escape the powerful pull of a black hole.

It is speculated that the Replicators acquired their knowledge from many races, and by pooling it together, they were able to surpass all other living species. It is also possible that they have encountered other races, prior to encountering the Asgard, whose technology was assimilated as well. This would explain how the Replicators escaped from their own galaxy and into that of the Asgard without encountering any other races within the Milky Way galaxy. Their prime source of knowledge however would most likely be the Asgard, who are extremely powerful in their own right. A human-form Replicator has also mentioned that the Replicators have a fair understanding of Ancient technology as well, mostly taken from databases on Asgard ships.
A "Mother Bug" creating new blocks. Replicators are driven solely by their desire to create more of themselves, and their intergalactic storm of conquest was originally based solely upon this desire. Replicators can form raw materials into new blocks, although the quality of the replicated blocks is based in part on what raw materials are available.  This base programming, to multiply at all cost, had led Teal'c to note that the Replicators are no more "evil" than a virus.

All Replicators exhibit the ability and desire to rapidly absorb any superior technologies they encounter, often passing up familiar or less advanced technology in order to acquire more advanced technology. They incorporate new alien technology into their currently existing store of knowledge, and constantly improve their combat and scientific abilities as they do so.

After the gynoid Reese, who had created the first generation of Replicators many centuries ago, was discovered and eventually killed by SG-1, her remains were given to the Asgard. The Asgard used the body to lure every Replicator in the known universe to Halla, an abandoned Asgard planet, as part of an elaborate plan to trap them inside a time-dilation field. However, after the Replicators came in contact with Reese, specifically the nanites used in her self-repair mechanism, they were able to create nanite-based blocks, eventually leading to the creation of human-form Replicators.  These nanites were based on neutronium, a material essential to the creation of human-form Replicators.

These Replicators, who were physically indistinguishable from ordinary humans, were composed of millions of microscopic nanites. However, this made them even more powerful, because these Replicators were resistant to projectile weapons besides being impervious to energy weapons. The only way to destroy them was by using the Replicator Disruptor. They possess massive strength, are extremely agile, and self-aware. Instead of attacking their victims with Replicator Spray, human-form Replicators instead prefer to use either their massive strength, or restructure their hand into a sword with which to impale their foes. In addition, these Replicators can probe the mind of humans by inserting their hand into the victim's forehead.  Human-form Replicators possess the ability to reform themselves at any time, not only enabling them to heal but also to reshape body parts. They can also deactivate portions of their bodies, effectively enabling them to leave body parts behind. These Replicators are, like the standard Replicators, also connected to the Replicator datastream called The Link.

They were originally created to be leaders of the Replicator race, and at least three distinct human-form Replicators have been known to lead their species into battle.  Eight human-form Replicators have been encountered thus far.

It should be noted that while it has been stated that the Bug-Form Replicators require multiple blocks to replicate, no information has been given regarding human-form replication other than the requirement of neutronium.

In 2006, the Atlantis Expedition discovered what appeared to be a new form of human-form Replicators, who called themselves the Asurans, in the Pegasus Galaxy.

These human-form Replicators were the result of a failed Ancient experiment to fight the Wraith. Desperate for more effective means of fighting their nemesis, the Ancients created the predecessors to the Asurans to infiltrate and destroy the Wraith from within.

To that end, these self-replicating nanites were programmed with an aggression surpassing even that of the Wraith. As they grew in numbers, the nanites began to assume the form of their creators. Eventually, the Ancients decided to terminate their experiment, destroying them. A few nanites however, survived, and through their replicative abilities, they brought about a "second birth" — this time, not only taking the form of their creators, but also recreating their city on a far grander scale.

The Replicators/Asurans were originally supposed to be the main enemy in Stargate Atlantis before it was decided that the show would take place in the Pegasus Galaxy.

The Replicators were originally created by a gynoid (a female android) named Reese, who had been constructed by an advanced human culture in the Milky Way. Reese's personality resembled that of a spoiled young girl. She seemed to lack the ability to comprehend the consequences of her actions and understand evil in any meaningful way. She was also incredibly inattentive, quick to have temper tantrums, and difficult to reason with. She displayed severe mood swings that, coupled with her formidable physical strength, were often disastrous. All these childish characteristics may as well have been a manipulative intention to destroy Earth and infest it with the toys (her "children", in her terms), as O'Neill thinks pointing out that she lied from the start, and when she made the first Replicator on the base already gave it the acid "defense" spray.

Apparently, Reese created the first Replicators as "toys" under her direct control. But when the citizens of her world became aware of the capabilities of the Replicators, they were alarmed and claimed she was "built wrong". Out of fear of being destroyed, she instructed her toys to protect themselves and their creator (self-preservation) and to make more of themselves (replication). However, the Replicators soon became too numerous to completely control, and escaped her dominance. In the end, they destroyed all society on her world, presumably by converting every usable substance into Replicator blocks. Reese escaped the catastrophe by "going to sleep", putting herself in stasis.

Eventually, the Replicators discovered a way to leave this galaxy, and traveled to the Ida galaxy, home of the Asgard, who discovered them on an abandoned planet. When the Asgard realized the danger the Replicators posed, they tried to destroy the scourge, but the machines adapted to the weapons the Asgard used to destroy them.

The Replicators were first encountered by the SGC after a swarm of Replicators had infested Thor's ship and found the location of Earth. Thor had managed to sabotage the outbound beams but brought up Jack O'Neill to figure out a way to defeat them. Aided by Samantha Carter and Teal'c, they placed a naqahdah-enhanced bomb on the external hull, and used it to destroy the deceleration drive, causing the ship to go into an uncontrolled atmospheric reentry. Shortly before crashing, they beamed up the Stargate as a means of escape.  However, a single Replicator survived and infected a Russian submarine before being neutralized.

Over a year later, SG-1 discovered Reese on her barren homeworld. She was brought back to the SGC and reactivated. After a near-repeat of what happened to her planet on Earth, she was killed. Her body was given to the Asgard, and ultimately yielded the key to capturing the Replicators, as one key command first installed by Reese was still present in all Replicators that followed: "Come Forth."

The Asgard attemped to trap all Replicators in the known universe inside a time-dilation field where time was slowed down some 10,000 times. However, when the Replicators arrived and found Reese's body, they hailed her as their creator and began to construct "cellular" blocks based upon her self-repair mechanism. Using these nanites, the first human-form Replicators were created. These human-form Replicators were able to reverse the time-dilation device, accelerating time inside the bubble. It was the Replicators' intention to increase their numbers exponentially in what would seem a small amount of time to the rest of the universe. The discovery of Reese's body also brought about the realization of their true purpose, the replacement of all biological life in the universe with their own superior form of life.

Outside the bubble, the Asgard were only aware that their plan had failed. Desperate, the Asgard requested the aid of SG-1. The Replicators would surely defeat any Asgard attempt to enter the bubble and repair the time-dilation device, whereas the technologically primitive humans would not be seen as a threat by the Replicators.

By the time SG-1 had arrived to assist, six human-form Replicators had been created, and the entire planet was covered with Replicator blocks. SG-1 discovered that the initial flaw in Reese's base programming - her lack of any real humanity - was later corrected in the creation of Fifth. His humanity, however, was seen as a flaw by the others, and removed from the subsequent versions created.

SG-1 used Fifth's "flaw" against him to trap the Replicators on the planet. Appealing to his compassion and emotion, namely his attraction to Samantha Carter, they convinced him to repair the time-dilation device and leave the planet with them. Unfortunately for Fifth, Jack O'Neill had no intention of releasing a human-form Replicator into the universe, and instructed Carter to rig the machine to trap Fifth as well.


It took the Replicators almost two years (in realtime, several days from their perspective) to escape. They used the Time-Dilation Device to counter the effects of the black hole, which was created by the Asgard by collapsing Halla's Sun. Still infatuated with Samantha Carter, Fifth abducted her and repeatedly tried to convince Carter to stay with him.

The Replicators then attempted to conquer the new Asgard homeworld Orilla, but an early warning from Thor alerted the Asgard and they were ready. As the Replicator ship emerged from hyperspace, it was immediately attacked by Asgard ships and destroyed. However, countless Replicator blocks rained down on Orilla, where they quickly began to replicate. It was then revealed that Fifth had broken off from the main Replicator ship as soon as it exited hyperspace and started taking control of Orilla but he was soon stopped by SG-1, and their recently constructed Replicator Disruptor. Because of the threat the Replicator Disruptor posed, Fifth threatened to kill Major Carter if SG-1 continued to destroy any more Replictors, he then retreated to a Replicator Spider Ship and took off into hyperspace.

Fifth remained the only human-form Replicator left in existence, and assumed leadership of the Replicator race. However, his humanity also made Fifth much more dangerous than any other Replicator in existence, who have no real concept of cruelty, torture or betrayal and were only concerned with self-replication. Fifth eventually accepted that he could never have a relationship with Sam in the form he desired, so he released her while on Orilla and then proceeded to created a perfect replica of her intended to accept him and govern the Replicator race with him. However, the so-called RepliCarter was identical to the original, even down to the very last memory and character-facet, what in part led to her despising what he was trying to do as much as the original Samantha Carter would have.

Due to Fifth's training of RepliCarter, Samantha Carter's doppelgänger had become a megalomanic, ruthless, dominating conqueror. Bent on finding a defense against the Replicator Disruptor, the only threat to her plans to dominate the universe, and to rid herself of Fifth, Replicator Carter devised an intricate plan, which involved sending RepliCarter to the SGC, claiming that she had betrayed and fled Fifth, and expressing a wish to be destroyed by their Disruptor. SG-1 took her to the Alpha Site to question her, where she informed them (untruthfully) that Fifth had immunized every single Replicator to the Disruptor, except her due to his spite. However, she promised to help them realign the Disruptor to overcome around Fifth's immunization.

As they began work on the Disruptor, RepliCarter was able to study the weapon to discover how to immunize her race from it. She relayed reports of her progress back to Fifth, keeping in communication with him via the datastream between all Replicators. Unbeknown to the SGC and SG-1, Fifth was tracking her position, on his way with one of his powerful Replicator ships to attack the planet, confident RepliCarter would upload the cipher to render the Disruptor useless before he arrived.

However, RepliCarter eventually betrayed Fifth by firing the Replicator Disruptor at his ship before she uploaded the cipher. Both he and his ship were destroyed, before RepliCarter immunized herself and her race. As she left the Alpha Site via its Stargate, Teal'c tried to stop her and forced her to leave her arm behind. The arm, decomposed into its deactivated cells, were studied by Carter, who felt responsible for her doppelgänger's victory. RepliCarter subsequently took over Fifth's position and became the leader of the entire Replicator race.

Several months later, Replicator Carter led her army back to the Milky Way, and launched a massive attack on the Goa'uld, taking over their territories and incorporating their technology into her own, the rather outclassed Goa'uld losing ships almost as quickly as they sent them out to confront her fleets, which captured them and rapidly turned them back against the Goa'uld.

Daniel is captured by RepliCarter. Knowing of the ascended knowledge still present in Daniel Jackson's subconscious mind, information that could possibly be a threat to her invasion, RepliCarter captured her doppelgänger's teammate. After hours of probing his mind, RepliCarter discovered the existence of the Dakara Superweapon, a device that, when altered, could be used to destroy her entire army at once.

However, the Replicator invasion, seen by many Jaffa as holy retribution for their blasphemy against their Gods, proved not only to be disastrous for the Goa'uld, but also caused many rebelling Jaffa to revert to their old religion. Teal'c and Bra'tac, along with many Free Jaffa, subsequently attempted to take over the Temple at Dakara, an act that they believed would prove to all Jaffa that the Goa'uld are not gods.

RepliCarter sent her forces to Dakara to eradicate the weapon, engaging the Jaffa rebels and Ba'al's fleet, who had turned his fleet on Dakara to drive out the Rebel Jaffa. However, Samantha Carter had also discovered the weapon, and she recognized it as the only thing capable of destroying the Replicators, being based upon the same technology as the Replicator disruptor.

However, the Replicators eventually landed on Dakara and attempted to reach the weapon. In the meantime, Daniel, whilst allowing RepliCarter to enter his mind, was able to work his way into her mind, and was able to momentarily freeze every single Replicator in the galaxy, including those on Dakara and those invading the SGC. This gave Samantha Carter enough time to alter the weapon with help from Jacob Carter/Selmak and Ba'al to send out waves that would only cause Replicators to permanently disassemble. This wave was spread over the entire galaxy via the Stargate, destroying all known Replicators, including RepliCarter, in the Milky Way galaxy.