Effects of host age and temperature on interspecific competition between Cotesia plutellae (Kurdjmov) and Oomyzus sokolowskii (Kurdjmov) were investigated in the laboratory, by exposing the 2nd, 3rd, or 4th of instar larvae of Plutella xylostella (L. ) to and by exposing the 3rd instar larvae at 20 [degree] C, 26 [degree] C, 28 [degree] C and 32 [degree] to the two parasitoids concurrently, or to one of two species at a time. When the 2nd and 3rd instar larvae were exposed to the two parasitoids concurrently, the parasitism rates by C. plutellae were significantly higher than those by 0. sokoloivskii. As compared with those exposures to one species at a time, the parasitism rates by C. plutellae were not reduced by the presence of 0. sokolowskii while those by 0. sokolowskii were significantly reduced by the presence of C. plutellae. No significant differences were found between the parasitism rates done by the two species together and those done by either one of them alone when the 2nd instar larvae were exposed. However, the parasitism rate by the two species together was significantly higher than those by either one species alone when the 3rd instar larvae were exposed. When the 4th instar larvae were exposed to the two species of parasitoids concurrently, there was no significant difference between the parasitism rate by C. plutellae and that by 0. sokolcnvskii, however, the parasitism rate by the two species together was significantly higher than that by C. plutellae alone, but not significantly higher than that by 0. sokolowskii alone. When the 3rd host larvae were exposed to the two parasitoids concurrently, the parasitism rates by C. plutellae were significantly higher than those by 0. sokolowskii, under all four tested temperatures. Parasitism rates by C. plutellae did not differ but those by 0. sokolowskii were significantly reduced, compared with those exposures of host larvae to either one species at a time. These results suggest that host stage is a more important factor than temperature in affecting the competition between these two species of parasitoids. Biometrics; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Competition; Abiotic factors; Physical factors Plutella xylostella (Plutellidae); Age; Hymenopteran parasites; Cotesia plutellae & Oomyzus sokolowskii; Parasite competition, effects of temperature & host age; Temperature Cotesia plutellae (Braconidae ); Oomyzus sokolowskii (Eulophidae ); Lepidopteran hosts; Plutella xylostella; Parasite competition, effects of temperature & host age; Interspecific competition; Temperature none