Composition of cellular fatty acids in methanol-utilizing bacteria was examined from the viewpoint of taxonomy. This group of bacteria showed a rather simple composition of cellular fatty acids, and was divided into two major groups on the basis of this composition. Pseudomonas methylotropha NCIB 10510, 10511, 10512, 10513, and 10515 showed the presence of a large amount of straight-chain saturated and unsaturated acids of C_16:0 and C_16:1, a small amount of straight-chain 3-hydroxy acid of C_10:0, and a minor amount of other acids. Other methanol-utilizing bacteria, Pseudomonas extorquens NCIB 9399, Pseudomonas spp. NCIB 9133, 9686, and 10597, and Protaminobacter rubes NCIB 2879, showed the presence of a large amount of straight-chain unsaturated acid of C_18:1, and a minor amount of other acids. Almost identical fatty acid composition was obtained from Microcyclus polymorphum NCIB 10516 and Hyphomicrobium variabile NCIB 10517, with the exception of the presence of cyclopropane acid of C_19:0 and other unknown acids. It was noteworthy that almost identical fatty acid composition was found in the cells grown on methanol-containing media and in the cells grown on media not containing methanol, in the both groups of methanol-utilizing bacteria.