Stylar proteins involved in the self-incompatible (SI) response of Lycopersicon hirsutum have been identified and mapped to the locus that controls SI (S locus). L. esculentum, a self-compatible (SC) species of cultivated tomato, does not display these proteins. Hybrids between SC L. esculentum and SI L. hirsutum are self-sterile despite these individuals bearing pollen containing the S allele of L. esculentum. In progeny derived from backcrossing the hybrids to L. esculentum, there was a strong correlation between the presence of the S allele from L. hirsutum and self-infertility. However, this relationship was uncoupled in a number of backcross (BC) progeny. The SI response appeared to be nonexistent in two self-fertile BC individuals that were heterozygous for the S allele of L. hirsutum, based on Mendelian segregation of a tightly linked DNA marker, CD15, in selfed progeny. Among these progeny self-fertile individuals that were homozygous for the L. hirsutum allele of the linked marker were also determined to be homozygous for an S-related proteins are not sufficient to elicit a self-incompatible response in L. esculentum and that there is a mutation(s) in L. esculentum somewhere other than the S locus that leads to self-compatibility.