none The insemination capability of males of the parasitoid wasp, Cotesia glomerata L. (Hymenoptera: Braconidae) was investigated under laboratory conditions. When single males were kept with five or ten virgin females, most males were able to inseminate almost all females within 1 day. However, when kept with 20 females, no males were able to inseminate all accompanying females within one day. The mean number of inseminations per male was 14, and the number of inseminations was not affected by male weight nor by the sex ratio of the cluster from which the male emerged. When newly emerged adults were kept with cocoon clusters from which they had emerged, on average nearly 75% of females were inseminated within 1 day; in some clusters, all females were successfully inseminated. Within a cluster, the number of inseminated females per male was not influenced by male weight nor by the sex ratio of the cluster. Therefore, each male of C. glomerata was considered to be equally capable of inseminating females, regardless of his origin. When males were offered ten virgin females daily, the number of inseminations per day began to decrease within a few days. No males lived more than 2 weeks, and for many males the total number of inseminations during their lifetime exceeded 30. A male's capability to inseminate females highly exceeded the common sex ratios of cocoon clusters and was considered to reflect the mating structure of this species. Biometrics; Reproduction; Reproductive behaviour; Ecology; Population dynamics; Population structure Cotesia glomerata (Braconidae); Weight; Mating; Insemination capability; Reproductive productivity; Male insemination capability; Weight & population sex ratio effects; Population sex ratio