The complex incompatibility system, characterized by a cooperative control of the individual S specificity by two complementary allelic series, was previously considered to have a duplicative origin. Its necessary building-stone was deemed to be a kind of S gene characterized by lack of allelic interaction. This kind of gene seemed to be an almost unique characteristic of the monocotyledons. Therefore, it seemed logical to expect the complex S gene system to be of an evolutionary young date and restricted to the monocotyledons. Recent investigations in the families Ranunculaceae and Chenopodiaceae reveal, however, complementary S gene systems of hitherto unknown complexity, involving at least three or four genuine S loci. These families presumably belong to a dicotyledonary evolutionary branch with ancestral basis shared with the monocotyledons. Most probably a complex type of incompatibility control was already present at this common ancestral basis. The findings may shed new light on the evolutionary relationships between the one-locus and the complex S gene systems.