Lipidic polyketides: We examined the in vitro reactions catalyzed by RpsA, a bacterial typeIIIpolyketidesynthase (PKS) from Rhodospirillum centenum. RpsA is the first typeIII PKS shown to be able to efficiently accept two molecules of extender methylmalonyl‐CoA and to synthesize tetraketide compounds through aldol condensation induced by methine proton abstraction.