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The Protozoan is an inoffensive creature seen on Xen.


The Protozoan appears as a bluish, transparent orb-like entity orbiting above the ground, with a smaller reddish sphere inside, also transparent and turning on itself. It has no AI, cannot be destroyed, and does not react to the player's or any NPC's presence.

It is first seen in Half-Life for a few seconds during the Resonance Cascade, orbiting behind two Bullsquids drinking from a Healing Pool. It is also seen in the same map in Opposing Force (reached several times with the Displacer Cannon) as well as Decay, at the very start of the bonus chapter, Xen Attacks.

Behind the scenes[]

The "protozoan" (Gr. "prôtos", "first" + "zôon", "animal"), also named "protozoon", actually exists. It covers any of a large group of single-celled, usually microscopic, eukaryotic protist organisms commonly living in water, such as amoebas, ciliates, flagellates, and sporozoans.[1] Protozoa can be plants, fungi, or animals. Thus, the Xen Protozoan can be considered as an extremely enlarged aerial protozoan.


  • "Protozoa", the model name, is actually the plural form.
  • When viewing its model in a model viewer, it is completely opaque. Its transparency only appears in-game.


List of appearances[]


  1. Protozoan on The Free Dictionary