The pollen–stigma interaction of self-incompatibility in Brassica species is controlled by a single genetic locus, the S locus. This locus encodes the S locus specific glycoproteins of the stigma. Hybridization of restriction enzyme digested genomic DNA isolated from homozygous strains of Brassica with cDNA probes encoding these glycoproteins yields a pattern of multiple fragments of varying size and intensity. The presence of S sequences as multiple related copies in the Brassica oleracea genome has been verified by the cloning of several different genomic regions containing S homology. Probes capable of distinguishing between S gene copies have been obtained and have demonstrated that two such copies are expressed. Characterization of additional probes specific for S gene copies will permit the study of the expression of the other copies, and the analysis of the genomic organization of the self-incompatibility multigene family.Key words: self-incompatibility, Brassica, cDNA probes.