Cotesia sesamiae (Cameron) is an indigenous larval endoparasitoid that attacks mid- to late-stage gramineous stem borer larvae in Africa. Two biotypes of C. sesamiae have been reported with differential abilities to suppress the immune system of Busseola fusca Fuller. Eggs of a C. sesamiae population from Mombasa were encapsulated, whereas eggs of a population from Kitale were not. Total and differential hemocytes were counted in larvae of B. fusca at six times (2 h, 14 h, 24 h, 72 h, 120 h, 168 h) after being exposed to parasitoids from Kitale and Mombasa. The total numbers of hemocytes in a larva parasitized by the C. sesamiae population from Mombasa were higher as compared with larvae parasitized by the C. sesamiae population from Kitale. Plasmatocytes, in particular, were reduced in larvae parasitized by C. sesamiae from western Kenya from 72 to 168 h after oviposition. Our results suggest that plasmatocytes probably play an important role in the immune response of B. fusca. Significant proportions of the host larvae were still at the larval stage for those parasitized by C. sesamiae from Mombasa and another species of Cotesia, Cotesia flavipes Cameron at day 12. A reduction of B. fusca larval weight was observed on day 12 after oviposition by C. sesamiae from Kitale.