Among the Hymenoptera, single locus sex determination (sl-CSD) describes a mode of sex determination in which females develop from fertilized eggs that are heterozygous at a single polymorphic sex locus. Males are haploid when they develop from unfertilized eggs or diploid when they develop from fertilized eggs that are homozygousat the sex locus. These diploid males are commonly inviable or sterile. This genetic load associated with sl-CSD is especially severe under conditions of inbreeding or low diversity of sex alleles. Since the 1970's it has been hypothesized that sl-CSD might be a specific case of multiple locus CSD (ml-CSD). Under ml-CSD, diploid males would develop only from fertilized eggs that are homozygous at each of two or more sex loci. Ml-CSD thus results in much lower fractions of diploid males than sl-CSD and can therefore significantly reduce the genetic load on a population. We have now found evidence for ml-CSD in the parasitoid wasp Cotesia vestalis (= C. plutellae). This conclusion is based on patterns of sex ratio and fractions of diploid males over multiple generation of inbreeding.