Two mitotypes of Busseola fusca (Fuller) (Lepidoptera: Noctuidae) namedKI and KII, co-exist in Kenya. Individuals of KII are more widely distributedthan those of KI. The present study assessed whether this was dueto differences in their reproductive potential and/or in their resistance tothe braconid Cotesia sesamiae Cameron, which is the most common larvalparasitoid of B. fusca in the region. Two populations of the parasitoid,one from the coastal and one from the inland regions of Kenya, whichdiffer in their ability to develop in B. fusca, were tested. Virgin KIIfemales started to call sooner during the night than KI females. Femalefecundity and egg viability were significantly lower for the heterogamousthan the homogamous crosses. Cotesia sesamiae from the inlandproduced larger progeny in KI than in KII host. Cotesia sesamiae from thecoast did not develop in either host. Despite their long time co-existencein the same geographical area, KII and KI conserved biological differencesin terms of time of calling, fecundity, fertility and resistanceagainst the larval parasitoid, C. sesamiae. This might explain the widerdistribution of KII as compared to KI in Kenya.