A successful colony of Glossina morsitans orientalis Vanderplank was initiated at Langford in early 1967 with 94 females imported as pupae from Rhodesia. In September 1969, further pupae were received from Rhodesia. Experiments were undertaken in which the four possible combinations of crosses were made between males and females from the Langford colony and from the 1969 importation from Rhodesia. Females of Langford origin performed equally well irrespective of the origin of their mates and it is concluded that there is no evidence that the Langford colony of G. morsitans is either performing abnormally well, because of adaptation to the laboratory, or abnormally badly, because of adverse effects of inbreeding. The poorer performance of females of Rhodesian origin was manifested in poorer survival rather than a lower fecundity; it is attributed to adverse effects on these flies caused by handling of the pupae in the field and/or the stress of the journey from Rhodesia to Langford. It would seem that to avoid any risk of inbreeding in laboratory colonies, wild males only should be introduced periodically.