Anaerobic bacteria are a promising new source for natural product discovery. Examination of extracts from the obligate anaerobe Clostridium roseum led to the discovery of a new family of natural products, the clostyrylpyrones. The polyketide synthase-based biosynthetic mechanism of clostyrylpyrones is further proposed on the basis of bioinformatic, gene knockout, biochemical analysis, and heterologous expression studies.