Abstract: Interspecific competition between Diadegma semiclausum Hellen (Hym.: Ichneumonidae) and Oomyzus sokolowskii (Kurdjumov) (Hym.: Eulophidae) was investigated at 25℃ in the laboratory, by exposing the third instar larvae of diamondback moth, Plutella xylostella (L.) (Lep.: Plutellidae) to either species alone or the host larvae already exposed to one species to the other. Both D. semiclausum and O. sokolowskii could lay eggs into the host larvae which had previously been parasitized by other species, leading to occurrence of multiparasitized hosts. When host larvae were exposed first toD. semiclausum and then to O. sokolowskii in time intervals of 0-4 d, ensuing parasitoid adults from the multiparasitized host larvae were nearly always D. semiclausum. When host larvae were first exposed to O. sokolowskii and then to D. semiclausum, the ensuing parasitoid adults from the host larvae were still always D. semiclausum when the oviposition of the latter took place within 1 d of that by the former parasitoid, but the proportion of O. sokolowskii adults increased as the time intervals between the two exposures increased and came to one hundred percent when the oviposition of the latter took place 3 d after that by the former parasitoid. These results suggest that competition for food resource between these two parasitoids may occur directly, and the final outcome of the competition depends on the order and interval of the two ovipositions.