Several bacterial strains that are obligate for both thermophily and hydrocarbon utilization have been isolated from a number of thermal and non-thermal environments. Mud and water samples obtained from geographic sites across the United States were subjected to enrichment procedures at 60° C with n-heptadecane as sole growth substrate. Organisms forming very small white colonies on agar surfaces were often evident on primary enrichment. These bacteria were Gram negative, aerobic, small, and rodshaped. They lacked pigmentation, motility, and the ability to form endospores. Growth occurred in the temperature range from 45° C to 70° C with the optimum around 60° C and at a pH near neutrality. Only n-alkanes from 13 to 20 carbons in length were utilized by these organisms as growth substrate. The mol% guanine plus cytosine values for these strains were between 68 and 70%. The physiological and morphological characteristics of these organisms are distinctly different from any previously described thermophilic microbes. It is proposed that they be placed in a new genus, Thermoleophilum gen. nov. with the type species being Thermoleophilum album gen. nov., sp. nov. The type strain in ATCC 35263.