This paper surveys the results obtained from the investigation of the homomorphic self-incompatibility polymorphisms in 19 species of flowering plants in terms of the number and frequency of alleles in their populations, the distribution of alleles between these populations and the total number of alleles in the species, and assesses the extent to which these results conform with theoretical expectations. The survey shows that, contrary to theory, the populations of most species do not contain a very large number of alleles and the same appears to be true for the total number of alleles in the species. Populations of Trifolium repens and T. pratense, however, appear to possess a large number of alleles for reasons which are unknown. Data on the distribution of alleles range from complete overlap to considerable differentiation between populations in respect of the complement of alleles they contain. The frequencies of alleles in populations of most species with gametophytic systems of self-incompatibility appear to be equal, as the theory suggests; frequencies in populations of Papaver rhoeas and Lolium perenne, however, appear to be significantly unequal. Allele frequencies in populations of sporophytic species are, as expected, unequal, those of dominant alleles being lower than those of recessive alleles. The implications of these results are discussed and the need for further, more comprehensive information on the attributes of these polymorphisms in natural populations is stressed.