The X Parasites are a fictional species from the video game Metroid Fusion. In their natural form, the average X Parasite is a hovering, volleyball-sized amoeboid creature that glows yellow, green, or red. A mutated blue variety also exists. Sometimes several such parasites will combine into a larger, more powerful form. The X Parasite mutates easily in order to adapt new environments. X Parasites have the unique ability to infect, copy the DNA of, and kill another creature, then use the stolen DNA to create a new body with the same form and behavior as the previous host. When injured badly enough, an X Parasite will revert to its amoeboid form and is extremely difficult to harm except by the Metroids' energy draining ability. X Parasites reproduce by asexual division.

Strictly speaking, the X are parasitoids rather than parasites. In a parasitic relationship, the host is not generally killed, whereas in a parasitoid relationship, the host is usually killed after the full development of the other organism, which is the case with the X.

The X Parasites in the Metroid storyline

The X Parasites are such prolific breeders that on their home planet of SR 388 they threatened to destroy the ecosystem. The Chozo stepped in and created the Metroids in order to keep the X Parasites' numbers in check. When series protagonist Samus Aran exterminated all of the Metroids on SR 388 in Metroid II, the X Parasites' population quickly made a comeback. They provided the main antagonist role in Metroid Fusion, particularly in the form of the SA-X, an X mimic of Samus Aran and her armorsuit.