Gamma irradiation was used to induce mutations affecting self-incompatibility (SI) in diploid Ipomoea trifida. Plants (S₁ homozygote) with developing flower buds were exposed with gamma rays of 10 and 20 Gy. Mature pollen from the irradiated plants was used for crosses with S₃ and S₁₀ homozygotes. By pollination test and Southern blot analysis of the M₁ progenies, we screened 2 mutant plants carrying a deletion of the S locus derived from irradiated pollen, designated ΔS₁. In both mutant alleles, the deleted region was found to exceed a genetic distance of 12 cM around the S locus. Progeny test of both mutant plants showed that neither female nor male gametes carrying the ΔS₁ allele were fertile probably due to the lethal effect of the chromosomal deletion spanning the S locus.