The chalcid, Oomyzus sokolowskii Kurdjumov has been recorded in many parts of the world as a major larval-pupal, gregarious endoparasitoid of the diamondback moth, Plutella xylostella (Linnaeus), a serious pest of brassica vegetable crops worldwide. This study investigated intraspecific variation between two populations of O. sokolowskii, one from Cape Verde Islands, West Africa and the other from Hangzhou, China. In all crosses and backcrosses between the two geographical populations, the numbers of progeny and sex ratio of progeny 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 and survival of immature stages. Observations on the interactions between the two O. sokolowskii populations and Cotesia plutellae (Kurdjumov), another major parasitoid of P. xylostella, showed that neither population could achieve successful parasitism of P. xylostella larvae already parasitized by C. plutellae. However, both O. sokolowskii populations could achieve hyperparasitism by ovipositing into a mid-late stage larva of C. plutellae developing inside the primary host. Contrary to earlier reports, no evidence of intraspecific variations in ability to hyperparasitize between these two populations of O. sokolowskii was found. Reproduction; Life cycle and development; Genetics; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Competition; Abiotic factors; Physical factors; Land zones; Palaearctic region; Eurasia; Asia; China; Oceanic islands; Atlantic Ocean islands Plutella xylostella (Plutellidae); Hymenopteran parasites; Cotesia plutellae; Interaction with Oomyzus sokolowskii (Hymenoptera) during parasitism; Oomyzus sokolowskii; Lack of intraspecific variation in parasitoid; Cape Verde & Zhejiang; Zhejiang; Hangzhou; Cape Verde; Lack of intraspecific variation in hymenopteran parasitoid & parasitoid species interaction Cotesia plutellae (Braconidae); Hymenopteran parasites; Oomyzus sokolowskii; Lepidopteran hosts; Plutella xylostella; Interaction with Oomyzus sokolowskii (Hymenoptera) parasitoid during host parasitism; Cape Verde & Zhejiang; Hyperparasitism; Interspecific competition; Zhejiang; Hangzhou; Cape Verde; Interaction with hymenopteran parasitoid during lepidopteran host parasitism Oomyzus sokolowskii (Eulophidae); Reproduction; Development; Population genetics; Hymenopteran hosts; Cotesia plutellae; Lepidopteran hosts; Plutella xylostella; Parasitoid geographical populations reproductive compatibility & lack of biological variation; Cape Verde & Zhejiang; Hyperparasitism; Survival; Interspecific competition; Temperature; Zhejiang; Hangzhou; Cape Verde; Lack of intraspecific biological variation & reproductive compatibility, lepidopteran host none