Apparent fundamental differences between monocot and dicot incompatibility systems raised the question whether these systems had developed independently, or whether they had a common origin. The incompatibility system in the dicot Ranunculus acris was examined because the family, Ranunculaceae, presumably has an evolutionary basis in common with the monocots. Studies in F1-families have shown that at least three multiallelic S-loci govern the incompatibility system. Further, complementary interaction between loci must be assumed in that all S-genes of the haploid pollen have to be matched in the pistil to cause incompatibility. These assumptions were confirmed by F2-families, which also showed that the S-loci imprint pollen gametophytically. The system found in R. acris differs from the systems normally found in the dicots, but is in agreement with the incompatibility system found in the monocot family Gramineae, although the latter system seems to be governed by only two S-loci. Evolutionary aspects are discussed, and essential questions which are still to be answered, are mentioned.