The eight natural avermectins produced by Streptomyces avermitilis have the carbon skeleton of either isobutyric or S-2-methylbutyric acid incorporated into their structures. A mutant of S. avermitilis has been isolated that contains no functional branched-chain 2-oxo acid dehydrogenase activity. The mutant, in contrast to its parent, is unable to grow with isoleucine, valine and leucine as carbon sources. In medium lacking both S(+)-2-methylbutyric and isobutyric acid, the mutant is also incapable of making the natural avermectins, while supplementation with either one of these compounds restores production of the corresponding four natural avermectins. These facts indicate that in S. avermitilis the branched-chain 2-oxo acid dehydrogenase enzyme functions not only to catabolize the cellular branched-chain amino acids in order to meet energy and growth requirements but also to provide the small branched-chain organic acid precursor molecules necessary for avermectin biosynthesis. Supplementation of the mutant strain with R(-)-2-methylbutyric acid yields novel isomeric avermectins unseen in the (unsupplemented) wild-type strain. It was also concluded that acetate and propionate production by branched-chain 2-oxo acid degradation is not absolutely essential for avermectin production.