The fatty acid composition of Bordetella pertussis (13 strains), B. parapertussis (3 strains), B. bronchiseptica (6 strains), Brucella abortus (6 strains), B. melitensis (4 strains) and B. suis (5 strains) was determined. Both genera contained straight-chain saturated and mono-unsaturated acids as well as cyclopropane substituted isomers, but the overall differences between the two genera were distinct. The Brucella species contained exclusively C16 to C19 acids. Minor amounts of hydroxylated acids (tentatively identified as 3-hydroxy-hexadecanoate and 3-hydroxy-octadecanoate) could only be detected after thin-layer chromatography and substantial concentration. The Bordetella species, on the other hand, contained a number of both non-hydroxylated and hydroxylated fatty acids of chain-lengths varying from C10 to C19. Intra-generically, no clear distinction between the Brucella species could be detected. The three species of Bordetella, on the other hand, were clearly distinguished by their fatty acid patterns. B. pertussis did not contain cyclopropane fatty acids encountered in significant amounts in the two other species, and B. bronchiseptica was characterized by containing 2-hydroxy-dodecanoate in significant amounts.