Division of labor: A combination of genetic, microbial, and chemical analyses solved the riddle of the dual epoxidation in the biosynthesis of rhizoxin, the causative agent of rice seedling blight. Bacterial endosymbionts of Rhizopus microsporus mediate the first epoxidation by a dedicated cytochrome P450 monooxygenase. The second oxirane ring is introduced by the fungal host and results in a substantially increased potency of the phytotoxin.