The presense of diploid males and a sex determination system in Diadegma semiclausum, a larval parasitoid of Plutella xylostella, was examined by using an allozyme as a genetic marker. Among six enzymes investigated, only phosphoglucomutase (PGM) was apparently polymorphic. Two discrete bands were detected on electrophoretic banding patterns of PGM, indicating PGM was a monomer with two alleles on one locus. After establishing pure lines of each PGM allele, the heterozygous females were allowed to mate with their sons to examine the occurrence rate of diploid males in their progenies. Thus the numbers of diploid males identified did not differ significantly from those expected from the single-locus complementary sex determination (CSD). Inbreeding and the subsequent emergence of diploid males could partly account for the male biased sex ratio in the laboratory culture of D. semiclausum.