Hydrocarbon-utilizing bacteria were isolated from oil-brine, soils etc. sampled in oil fields in Japan during 1956, and the following species were identified: Corynebacterium hydrocarboclastus nov. sp., 11 strains; Pseudomonas nitroreducens nov. sp., 1 strain; Pseudomonas maltophila Hugh and Ryschenkow, 5 strains: Brevibacterium lipolyticum (Huss) Breed, 2 strains; Pseudomonas desmolytica Gray and Thornton, 5 strains; Flavobacterium ferrugineum Sickles and Shaw, 1 strain; and Alcaligenes faecalis Chastellani and Chalmers, 1 strain. One difference between Gram-negative bacteria and Gram-positive bacteria was described on the basis of the ability of assimilating hydrocarbons.