Three main sex-determining mechanisms have been proposed for Hymenoptera : genetic balance, single locus (sl-CSD) and multilocus (ml-CSD) complementary sex determination. In the last two cases, sex is not determined by the number of chromosome sets but by heterozygosity at one or several loci. Individuals are male when hemizygous (haploid) or homozygous (diploid) at all sex-determining loci. Usually, this results in haploids developing as males and diploids as females, although diploid males can also appear, particularly under conditions of inbreeding. Aphidius rhopalosiphi (Aphidiineae : Hymenoptera Braconidae) is a cereal aphid parasitoid that can potentially be used as a biological control agent. Phylogenetic studies suggested that, within parasitoid wasps, the sl-CSD is present in both the Ichneumonoidea superfamily and the Braconidae family. Here, we directly test the sl-CSD model in A. rhopalosiphi by inducing diploid male production by brother-sister mating in laboratory-selected isofemale lines. Ploidy levels were analyzed with two complementary methods : DNA flow cytometry and DNA microsatellite markers. We observed a significantly male-biased sex ratio after sib mating, but no diploid males were detected by DNA analysis. The difference between the observed and expected sex ratio suggests that a sl-CSD model with two alleles may be applicable, which would imply that most diploid males are unviable in A. rhopalosiphi. Consequences of diploid male production are discussed in terms of the evolutionary biology of Hymenoptera and aphid biological control. Genetics Aphidius rhopalosiphi (Aphidiidae); Sex determination genetics none