We investigated the effect of mutual interference on the attack efficiency and the rate of successful parasitism on the parasitoid Spalangia cameroni (Perkins) attacking pupae of the stable fly Stomoxys calcitrans (L.). Female parasitoids (2, 4, 8, 16, or 32) were exposed to 100 fly pupae during 24 h. The number of pupae that were attacked and the number successfully parasitized increased with the parasitoid density and reached a maximum of ∼70 and 50, respectively. Parasitoid-induced mortality (PIM) was about 20 pupae, irrespective of parasitoid density. The per capita rates of attack, successful parasitism and parasitoid-induced mortality declined monotonously with parasitoid density. Progeny sex ratio was female biased for all parasitoid densities, but declined significantly with increasing parasitoid density from ∼70% females at the lowest density to ∼60% at the highest. Mutual interference was incorporated into a functional response model to predict the attack rate and the rate of successful parasitism at different temperatures, host densities and parasitoid densities. The model explained 93.5% of the variation in the observed number of attacked pupae and 91.5% of the variation in the number of successfully parasitized pupae. The model predicts that increasing parasitoid densities will increase the percentage of killed hosts, but only up to a certain density. Above this density, a further increase in parasitoid abundance will actually lead to a decline in the percentage parasitism. These findings may have some implications for using S. cameroni in biological control against flies using inundative releases.