Genetic signatures associated with adaptation to extreme conditions are of interest for understanding the limits of life on earth and in the search for extraterrestrial life, but animals are relatively rare in extreme habitats. We have performed sequencing and protein prediction on a unique extremophile metazoan, Halicephalobus mephisto, a nematode isolated from the Beatrix gold mine in South Africa, just over a kilometer below the earth’s surface. The deep subterranean environment subjects H. mephisto to high pressure, heat (up to 41 degrees Celsius), utter darkness, and low oxygen. Here we present data supporting an unusual evolutionary signature in its genome consistent with adaptation to an extremely warm environment. Specifically, a sixteen-nematode comparative analysis revealed an expanded repertoire of 70 kilodalton heat-shock protein (Hsp70) in H. mephisto, which contains over 100 detected Hsp70 gene paralogs as compared to just 16 in C. elegans. H. mephisto exceeds all sequenced nematodes (if not all sequenced organisms) in Hsp70 gene content, which is significant because Hsp70 proteins are specialized chaperones for re-folding heat-damaged proteins. In addition to a large number of Hsp70 proteins, we found evidence for 15 of these Hsp70 proteins being derived from bacterial sources, some of which are also extremophiles. These data are consistent with horizontal gene transfer as a mechanism of animal adaptation to the deep, hot terrestrial subsurface.