THE presence of high concentrations of hyperthermophilic archaea in Alaskan oil fields has been attributed to viable hyperthermophiles in low concentrations in the injected sea water, but the existence of an indigenous community within the reservoir was ruled out(1). Here we present evidence for the existence of indigenous thermophilic bacteria and hyperthermophilic archaea from a continental petroleum reservoir about 1,670 m below the surface. The thermophilic isolates were repeatedly obtained from different wells and thrived in media similar to conditions in the wells, suggesting that these isolates are members of a deep indigenous thermophilic community. The unexpected presence of marine hyperthermophilic archaea in a deep continental environment extends the known ecological habitat of this group of organisms, and their unusual coexistence with terrestrial bacteria suggests that thermophiles may be widespread deep in the crust of the earth.