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"Listen, you've got to get down below. There's something coming through and it's the nastiest-looking thing yet. Some of your buddies went down there a while ago, and I haven't seen them since."
Security Guard[src]

The Gene Worm is the main antagonist and final boss of Half-Life: Opposing Force, and the most mysterious of the Race X aliens. This life form is said to be the equivalent of a "biological resource management factory." It is suspected to be capable of taking any natural resource and assimilating it to make them useful to the aliens' needs. If one fully entered Earth and became productive, it would effectively alter the planet enough to become useful as a new homeworld for Race X. However, this was never shown in action.[2]


Physically, the alien resembles a green, worm-like being with many tentacles. It bears a striking resemblance to the Xen Tentacle, appearing to have green skin with many tentacles and possessing a beak-like protrusion similar in both shape and position.

The Gene Worm attacks with its tentacles and also ejects a deadly chemical stream from his mouth. Its outer hide is impervious to damage. In order to kill it, the player must blind it by shooting both eyes with the mounted laser guns on either side of the worm. This causes it to summon reinforcements by opening its abdominal cavity, which spawns a Shock Trooper. The worm takes damage from attacks hitting a void behind its stomach cavity. When the Gene Worm is defeated, it sinks back into his portal, but not before emitting a "portal storm" into the room, similar to the one that occurred as the Nihilanth died. When it is idle and no threat is visible, it will gently sway its tentacles around and emit soft hissing and groaning noises.

The Gene Worm is found deep underground in a seemingly abandoned industrial area of Black Mesa and appears through a portal opening powered by Sprites in a room there, apparently either unwilling or unable to fully enter this world, most likely because of the minuscule size of the portal behind it in proportion to the Gene Worm's presumably immense size and length. The room itself has been fitted with mounted weapons which are powered by Xen energy crystals. The player character, Adrian Shephard, is sent down to the portal by a Black Mesa security guard, who remarks "Some of your buddies went down earlier, and I haven't seen 'em since." These soldiers were only able to set up the mounted weapons before their unseen demise. They were presumably slain by the Gene Worm and devoured.


  • The Gene Worm somewhat resembles the fictional Shoggoths, the works of H.P. Lovecraft.
  • In the Opposing Force files, there is a sound file related to the Gene Worm called "dsbossit". When played, it sounds like plain gibberish. But when played backwards, it plays a distorted version of the sentence "To win the game you must kill me, Randy Pitchford". (Randy Pitchford is the CEO of Gearbox) This is a nod to Doom 2 where "to win the game you must kill me, John Romero" is played backwards when the final boss sees the player. Furthermore, dsbossit is the name of this sound in Doom 2's main WAD file.
  • There is a glitch where on some Linux and Mac systems, the gene worm will retreat too quickly upon defeat. After this, the portal storm will still happen but when the big portal appears, the player will not be teleported and will be unable to complete the game.
  • The Destroyer from Borderlands (another Gearbox game) bears an uncanny resemblance to Gene Worm. Both have vulnerable eyes, share their body type with multiple notable tentacles and are intent on devouring everything in their way.


List of appearances[]