A gene coding for isocitrate dehydrogenase (ICDH) was cloned from "Caldococcus noboribetus" and sequenced. An amino acid sequence of ICDH displayed similarities to bacterial ICDH genes found in both "Vibrio" and "E. coli" species. Sequences from the latter shared about 50% identity with the archaeal ICDH. The gene was expressed in "E. coli" by ligating it to a T7 promoter and the resulting molecular weight of the gene product was about 48,000. This estimate was consistent with an estimate from the deduced amino acid sequence. The gene product also displayed a NADP-dependency in order to function at 80 °C. The ICDH gene isolated from "C. noboribetus" demonstrated a higher thermostability than a host derived ICDH. From this information, it is plausible that this species could provide information regarding an ancestral hyperthermophile. Such information would help build a complete phylogenetic tree of hyperthermophilic archeabacteria.