Herbicidins are adenosine-based nucleoside antibiotics with an unusual tricyclic undecose core decorated with a (S-hydroxy)tiglyl moiety. Feeding studies are herein reported demonstrating that the tricyclic core is derived from n-glucose and D-ribose, whereas the tiglyl moiety, is derived from an intermediate of L-isoleucine catabolism. Identification of the gene cluster for herbicidin A biosynthesis in Stieptomyces sp. L-9-10 as well as its verification by heterologous expression in a nonproducing host are described, and the results of in vitro characterization of a carboxyl methyltransferase encoded in the cluster, Her8, are presented. Based on these observations, a biosynthetic pathway is proposed for herbicidins.