The Pit Worm's caterpillar-like segmented body supports a head dominated by a single large eye with two vertical eyelids, and a twelve teeth mouth under it. It is found in the Waste Processing Area 3's Toxic Disposal Basin, a large room partially flooded with water, where the creature came from the ground (like an earthworm, hence its name). Its body is likely longer than seen, although it is unknown how much.
 Behavior and skills
The Pit Worm's eye is its primary weapon, capable of firing a searing energy beam at nearby enemies. If enemies get too close, the worm also has large claws to strike out with. If it is shot in the eye, it will recoil, shielding it with its claws for a few seconds before striking aggressively at the player. The Pit Worm makes bird-like vocalizations. Like the Gene Worm, only one individual of that species is ever seen in-game.
If one manages to run into one of the many passages leading into other rooms, the Pit Worm will screech angrily and swipe viciously at the entrance of the passage taken.
In Opposing Force's seventh chapter, Vicarious Reality, Shephard picks up a radio transmission from a group of HECU Marines fighting the Pit Worm. In the next chapter, Pit Worm's Nest, Shephard reaches the Waste Processing Area 3 and finds all the soldiers dead (although many were likely killed by Pit Drones infesting the area), and the Pit Worm standing in the Toxic Disposal Basin. As traditional weapons only upset the creature without killing it, Shephard must use toxic waste to defeat it. To do so, he must activate the steam vent (located near the creature), the main pressure valve (also located near the creature, and to be accessed thanks to the hand-held Barnacle and Spore Launcher spores on the wall, since the ladder to access the valve is broken) and the primary pump gearbox (located in Waste Station 02, another area, and by using an emergency override button) to unlock the toxic waste arrival, then release the toxic waste with the waste flush button, flooding the room with toxic waste and killing the creature. After that, Shephard merely has to activate the bridge previously blocked by the Pit Worm, and cross the Toxic Disposal Basin room to proceed further, in sewers leading to ruined hangars in Sector G.
 Behind the scenes
- The Pit Worm obstacle is similar - albeit less challenging - to the Tentacle puzzle in the Half-Life chapter Blast Pit, in that the player cannot simply use his weapons to defeat the creature, but must use the surrounding environment.
- The valve and the gearbox to be activated are obvious references to Valve Software and Gearbox Software, left there as Easter eggs. The reference to Steam is likely a coincidence, as it did not exist back then.
- Another Easter egg are the nearby garbage compactors, especially the flooded one, which are directly designed after the Death Star's garbage compactor seen in the film Star Wars Episode IV: A New Hope.
- The Pit Worm's screams appear to be based on vulture sounds, while its other idle sound resembles certain vocalizations produced by red foxes.
 List of appearances
- Half-Life: Opposing Force TV Spot (First appearance) (Non-canonical appearance)
- Half-Life: Opposing Force
|Creatures||Gene Worm · Pit Drone · Pit Worm · Shock Trooper · Sprite · Voltigore|
|Weapons||Shock Roach · Spore Launcher|
|Technology||Teleportation · Transportation · Weapons|