Hydrocarbon‐degrading bacteria in freshly collected petroleum‐rich Guaymas Basin sediments were enumerated on mineral base media separately containing naphthalene, biphenyl, or the aromatic carboxylic acids benzoate, p‐hydroxy‐benzoate, mandelate, salicylate, phenylacetate, phthalate, or hydrocinnamate. The total numbers of bacteria were about 2 orders of magnitude higher on the carboxylic acid media than on the two aromatic hydrocarbons, averaging 105versus 103bacteria per gram of sediment. Of 151 isolates of aerobic, mesophilic marine bacteria, 124 grew exclusively on one or more of the aromatic carboxylic acids. The remaining 27 isolates used either naphthalene, biphenyl, dibenzofuran, toluene, or phenanthrene and various combinations of aromatic carboxylic acids. A similarity analysis of 135 of the isolates on the basis of substrate use, sodium chloride requirement, and growth on complex organic marine media identified 91 metabolically different bacterial strains: 21 belonging to the group of aromatic hydrocarbon utilizers and 70 to the group of carboxylic acid utilizers. Since compounds utilized by organisms of the first group represent end products of petroleum catagenesis, we expected these organisms to predominate. Our results indicate, however, that in Guaymas Basin sediments the majority of hydrocarbon‐degrading bacteria, numerically and metabolically, appear to be determined by the presence of significant quantities of aromatic carboxylic acids.