Fatty acids of Fusobacterium spp. were examined by GLC. F. nucleatum, F. necrophorum, F. mortiferum, F. gonidiaformans and F. varium showed similar patterns, characterized by the presence of 3-hydroxytetradecanoate, n-tetradecanoate, hexadecenoate, n-hexadecanoate, octadecenoate, n-octadecanoate and a component having the properties of octadecadienoate. F. nucleatum contained 3-hydroxyhexadecanoate as a disctinctive character. Simpler fatty acid patterns characterized bythe absence of 3-hydroxytetradecanoate and other hydroxy fatty acids were observed in F. plauti, the single strain of F. prausnitzii and in the majority of strains classified as F. russii and F. naviforme. Neither methyl-branched nor cyclopropane fatty acids could be detected in any of the strains examined. In addition to fatty acid methyl esters, the chromatographic profiles of all species except F. mortiferum, F. gonidiaformans and F. naviforme contained substantial amounts of fatty aldehyde dimethyl acetals of chain lengths C14-C18.