Diadromus collaris (Gravenhorst) (Hymenoptera: Ichneumonidae) has been recorded in many parts of the world as a major pupal endoparasitoid of the diamondback moth Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae). Experiments were conducted to compare reproductive compatibility and performance as affected by temperature between two geographic populations of D. collaris, one from Hangzhou, Zhejiang and the other from Lishan, Taiwan, China. The numbers and sex ratio of progeny in all possible crosses and backcrosses were similar to those obtained within each of the populations, demonstrating complete reproductive compatibility between the two populations. The two populations showed similar responses to temperature with respect to development time, adult longevity and number of host pupae parasitized per female. However, the population from Hangzhou achieved higher rates of survival from larva to adult emergence at high temperatures and had higher proportions of female progeny at some temperatures, than the population from Lishan. The Hangzhou population achieved higher intrinsic rates of increase at all temperatures tested from 15 to 33.5℃, and the differences were significant at 30 and 32℃. The differences between the two populations in ability to survive high temperatures may reflect their adaptation to local climates. The significance of the intraspecific variations revealed is discussed with regard to the use of geographic populations of the parasitoid in the biological control of P. xylostella.