The genetics of self- and cross-incompatibility was studied in 12 related families ofLesquerella densipilaand in three families of theF1hybridL.densipila×L.lescurii. Silique swelling was used as the criterion for compatibility. Incompatibility results from the failure of pollen tubes to penetrate the stigmatic surface. Incompatibility inLesquerellais controlled by a series of multipleS-alleles at one locus. The pollen phenotype is determined sporophytically. A minimum of eightS-alleles was obtained fromL.densipila, and two fromL.lescurii. Two sorts of gene interaction, independence and dominance, were found among the fully analyzed heterozygotes. Dominance was restricted to the pollen controlling part of the alleles and two levels of potency were established.