Fatty acids were extracted from saponified vegetative cells and myxospores of Myxococcus xanthus and examined as the methyl esters by gas-liquid chromatography. The acids consisted mainly of C(14) to C(17) species. Branched acids predominated, and iso-pentadecanoic acid constituted half or more of the mixture. The other leading component (11-28%) was found to be 11-n-hexadecenoic acid. Among the unsaturated acids were two diunsaturated ones, an n-hexadecadienoic acid and an iso-heptadecadienoic acid. No significant differences between the fatty acid compositions of the vegetative cells and myxospores could be detected. The fatty acid composition of M. xanthus was found to be markedly similar to that of Stigmatella aurantiaca. It is suggested that a fatty acid pattern consisting of a large proportion of iso-branched C(15) and C(17) acids and a substantial amount of an n-16:1 acid is characteristic of myxobacteria.