The suitability of Sesamia calamistis Hampson and Busseola fusca (Fuller) for the development of two geographical populations of Cotesia sesamiae (Cameron) was examined in the laboratory. One population of the parasitoid was collected from the coast of Kenya and the other from the inland. Both populations of the parasitoid could develop on S. calamistis. On B. fusca, the inland population of C. sesamiae was able to develop, while the population from the coastal area of Kenya was encapsulated. Mating studies revealed that the two parasitoid populations were partially reproductively isolated. Unidirectional incompatibility, possibly caused by the Wolbachia infection, was observed when males from the infected coastal population were mated with females from the uninfected inland population.