The interspecific interactions between Cotesia plutellae (Kurdjumov) (Hym.: Braconidae) and Oomyzus sokolowskii (Kurdjumov) (Hym.: Eulophidae) were investigated at 28 degrees C in laboratory, by exposing the 3rd instar larvae of diamondback moth, Plutella xylostella L. (Lep.: Plutellidae) to two parasitoids concurrently, or to either species alone, and by exposing the host larvae already exposed to one species to another species. Compared with those exposures of host larvae to either one species at a time, the parasitism rates by C. plutellae were not reduced by the presence of O. sokolowskii, but the rates of parasitism by O. sokolowskii were significantly reduced by the presence of C. plutellae, when the host larvae were exposed to the two species of parasitoid concurrently. No significant differences were found between the parasitism rates done by the two species together, and those done by either one of the two species alone. When host larvae were exposed to O. sokolowskii after exposure to C. plutellae, the successful parasitism rates by C. plutellae were unaffected, while the parasitism rates by O. sokolowskii were only 8-13%. However, O. sokolowskii could parasitize the old larvae of C. plutellae developed inside host larvae. Cotesia plutellae could parasitize the host larvae parasitized previously by O. sokolowskii, and some individuals of C. plutellae in these multiparasitized hosts could develop normally and survive to adult emergence. When O. sokolowskii wasps were exposed to healthy host larvae and host larvae parasitized by C. plutellae, they preferred to parasitize the healthy ones. These results suggest that C. plutellae is the stronger competitor, but its competition capacity will be reduced by the parasitism by O. sokolowskii.