A two-loci gametophytic incompatibility system is indicated in our experiments involving hybrids between S. phureja accessions and between S. stenotomum and S. phureja. The S locus is epistatic to the R locus. An allele Rfi at the latter locus when present in homozygous condition, leads to female incompatibility, independent of the genotype of the pollen. The two-loci system is assumed to have evolved from doubling the chromosome number of ancestral species with one locus for incompatibility. The irregularities observed in the studies are tentatively ascribed to recombining modifier complexes. These modifiers may be maintained in cultivated species owing to their preponderantly vegetative propagation.