The closely related tsetse fly subspecies Glossina palpalis palpalis (Nigeria origin) and Glossina palpalis gambiensis (Burkina Faso origin) hybridize readily in the laboratory. Hybridized G.p.palpalis females produced less offspring than the parental intrasubspecific crosses. Adult emergence was below 70% with at least 78% being females. Most female hybrids were fertile whereas most of the male hybrids were sterile when backcrossed to the G.p.palpalis parental line. All F1 males were capable of transferring a spermatophore but their mates rarely had sperm-impregnated spermathecae. Their testes rarely contained mature sperm; moreover, sperm, when present had low or no motility. During laboratory cage tests with virgin females of both subspecies and either sexually mature male G.p.palpalis or G.p.gambiensis, there was no indication for selective mating. The same was true when gamma irradiated males (120 Gy treatment in air) were used. In the latter case complete sterility was induced causing embryonic arrest in all inseminated female mates. Consequently, in ratio tests with untreated virgin G.p.palpalis females, untreated G.p.palpalis males and an increasing number of irradiated G.p.gambiensis males, there was a gradual decrease in production of viable offspring. The results of the present study are discussed with a view of using a combined hybridization and induced sterility in distinct geographical zones where the two subspecies are present.