This study investigated the development, emergence and mating of Diadegma semiclausum Hellen, all important parasitoid of diamondback moth in the laboratory at 21 [plus or minus] 1[degree]C and 50-60% RH. with a photoperiod of 12:12 h (light: dark). The parasitoids developmental period was significantly shorter and pupal weight significantly higher if their mothers parasitised the fourth instar larvae of DBM (P0.01)The offspring sex ratio was strongly male-biased (20%) but no significant difference was detected between host instars. Most adults emerged during the first few hours of the photophase and none emerged during the scotophase. Mating had no effect oil the longevity of both sexes. but small females lived significantly longer than the larger ones (P0.05). Male age and size had no effect oil mating Success, whereas small females achieved higher mating success than large one (P0.05). The implication of this study in mass-rearing in the laboratory is discussed. Diadegma semiclausum (Ichneumonidae): Parasite; Plutella xylostella (Plutellidae): Host. Diadegma semiclausum: Rearing techniques, Mass rearing implications, developmental period, emergence and longevity, Age, Size, Mating, Reproductive productivity, Mating success, Development, Developmental period, emergence and longevity, Lepidopteran hosts, Plutella xylostella, Population sex ratio, Offspring sex ratio; Plutella xylostella: Hymenopteran parasites, Diadegma semiclausum, Parasitoid development and reproduction, larva. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans. none