nally methyl-branched fatty acids, 13-methyltetradecanoic (iso-C15) and 15-methylhexadecanoic (iso-C,7), in Bacillus subtilis (natto) (138), and 12-methyltetradecanoic (anteiso-C,5) acid in a species of Sarcina (4), was first reported. After these reports, extensive studies conducted in my laboratory led to the discovery of three additional branched-chain fatty acids; 12methyltridecanoic (iso-C14), 14-methylpentadecanoic (iso-C16), and 14-methylhexadecanoic (anteiso-C,7) acids in the lipids of B. subtilis (76). These branched-chain acids are grouped in three series, based on their biosynthetic relationships (28, 77, 79, 80, 92, 102, 119) (Fig. 1). The definitions of iso and anteiso permit only iso methyl-substituted fatty acids (an iso ethyl would be an anteiso methyl of one-higher chain number), but would allow both methyl and ethyl as anteiso substitutions. Only the methyl anteiso substituent has so far been found in 391