In nature, competing species often achieve coexistence through niche differentiation. We examined this phenomenon for Pachycrepoideus vindemmiae and Spalangia endius (Hymenoptera: Pteromalidae), two species of pupal parasitoids that are considered biological control agents of house fly, Musca domestica (Diptera: Muscidae). We examined the ability of each species, alone and in combination, to locate host pupae buried at different depths (0, 1, 2, 4, and 6 cm) in three types of substrate (sand, dry wheat bran, and spent fly diet). We then evaluated the competitiveness of each species by allowing first one species, then the other species, to parasitise host individuals within time periods ranging from less than 2 hours to 96 hours of each other. Spalangia endius exhibited greater ability than did P. vindemmiae to locate host pupae buried at depths below one centimetre. Conversely, P. vindemmiae exhibited a greater competitive ability, being more likely to emerge from pupae co-parasitised by S. endius, regardless of oviposition interval or sequence. Our findings suggest that these two parasitoid species coexist through niche differentiation. Our findings also indicate that to increase the effectiveness of biological control, the environmental conditions and risk of interspecific competition should be considered when selecting parasitoid species for release.