Some Hymenoptera have single locus Complementary Sex Determination (sl-CSD); heterozygous individuals are female (diploid fertilised eggs) and hemizygous individuals are male (haploid unfertilised eggs). Through inbreeding homozygous diploids can arise which develop into males that are often sterile and sometimes inviable. The phylogenetic distribution of sl-CSD is unclear. In this study, we used inbred crosses to demonstrate that the parasitoid wasp Venturia canescens (Gravenhorst) has sl-CSD. Diploid males were detected through heterozygosity at a marker locus (Virus Like Particle protein). They were fully viable at 20[degree], 25[degree] and 30[degree]C as the sex ratios of inbred crosses did not deviate from controls after adjustment for diploid male frequency. These results further confirm the existence of sl-CSD in the family Ichneumonidae. none