Halicephalobus mephisto is a species of nematode, among a number of other roundworms, discovered by geoscientists Gaetan Borgonie and Tullis Onstott in 2011.

[citation needed] Halicephalobus mephisto is resistant to a temperature as high as 37 °C (higher than most terrestrial nematodes can tolerate),[2] it reproduces asexually, and feeds on subterranean bacteria.

[2] It is named after Mephistopheles, the Lord of the Underworld in the Faust story,[2] and alludes to the fact it is found so deep under the Earth's surface.

[1] This nematode is able to thrive in such extreme conditions due to its adaptations to the environment including changed cellular respiration pathways to facilitate survival in low levels of oxygen,[4] an expansion in their stress-response gene families such as Hsp70 to aid in the protections against thermal damage,[5] and its modified cytochrome c oxidase enzyme that helps stabilize respiration and elevated temperatures.

Unlike other nematodes, it does not prefer Escherichia coli, and would rather feed on the sulphophile endolith and depth specialist Desulforudis audaxviator.