A Streptomyces sp. isolated from a shallow water sediment sample collected off Heron Island, Australia, afforded three new polyketide macrolactams, heronamides A-C (1-3). Structures were assigned to the heronamides on the basis of detailed spectroscopic analysis, chemical derivatization and biosynthetic considerations. A plausible biosynthetic pathway is proposed in which key carbocyclic ring transformations proceed via an unprecedented synchronized tandem electrocyclization. This biosynthesis provides a framework for the assignment of complete relative configurations across all heronamides, and inspires an attractive biomimetic strategy for future total syntheses. Heronamide C elicits a dramatic and reversible non-cytotoxic effect on mammalian cell morphology.