Males of the hymenopteran Diadromus pulchellus are normally haploid, but diploid males can be obtained by inbreeding. Inbred crosses within strains that are polymorphic at the enzymatic loci Pgm-2 and Ao-4 or that differ by a body colour mutation produce heterozygous diploid males in offspring. The genotypic distributions observed in such progeny were compared with expected results under the one-locus sex determination model or the two independent loci model. The results show that only the one-locus multi-allele model fits the data and allow a provisional estimate of similar to 15 sex alleles. These conclusions generalize the case of existence of a single locus to determine sex in Hymenoptera and are discussed at the evolutionary level. none