The fatty acid composition of a thermophilic Bacillus sp. was altered by the addition of isobutyrate, isovalerate, alpha-methylbutyrate, leucine, and isoleucine to the growth medium. With isobutyrate, 81% of the fatty acids had 16 carbon atoms and 79% were iso-fatty acids with an even number of carbon atoms. With leucine, 58% of the fatty acids had 15 carbon atoms and 86% were iso-fatty acids with an odd number of carbon atoms. With isoleucine, 72% of the fatty acids had 17 carbon atoms and 88% were anteiso-fatty acids with an odd number of carbon atoms. Thus, by altering the composition of the growth medium, cells were produced in which the majority of the fatty acids had either 15, 16, or 17 carbons and belonged to each of the three groups of branched-chain fatty acids. The wide variation observed in the fatty acid composition makes it unlikely that any specific branched-chain fatty acid is required for vital functions.