Pupae of Glossina morsitans Westw. collected in the Zambezi valley near Kariba, Rhodesia, were brought to the laboratory, and the effects of the chemosterilant tepa were investigated by observations on the resulting adults. In the standard test, 25 pairs of adults were caged together for 28 days at 79°F. and 70 per cent, relative humidity and fed on guinea-pigs, and the survival of both sexes, the numbers of pupae and adults produced and (in some tests) the insemination rate of the parent females were recorded. With untreated flies, the insemination rate was 94 per cent, or more, averages of 28 pupae and 26 adult progeny per cage were produced, and 72 and 77 per cent, of males and females, respectively, survived for the 28 days. These results were compared with those obtained when the flies of one sex in the cage had been treated with tepa either as pupae or as adults.

Injection of 1 μg. tepa into the thorax of young males produced complete sterility. Dipping the pupae in 5 per cent, tepa solution for one minute caused complete sterility of adults emerging during the first two post-treatment weeks and partial sterility of males emerging during the third week. Adults emerging from dipped pupae washed one day after treatment were fertile. Dipping pupae in solutions containing 1 or 0·5 per cent, tepa did not completely sterilise the resulting adults. Contact exposure of adult males and females of various known ages usually resulted in complete sterility (99·7%) after exposures ranging from 15 to 240 min. to deposits on glass of 10 or 50 mg./ft.2 tepa. Males exposed for 240 min. to 10 mg./ft.2 tepa retained their sterility throughout a 42-day test period and, in special tests, competed well with untreated males for the females, but their life length was reduced by 25 per cent, when unmated and by 33 per cent, when mated. Males exposed for 60 min. or less survived as well over a 42-day test period as untreated males, but those exposed for 15 min. recovered an undesirable degree of fertility. No treatment affected the ability of the male to inseminate the females. Sperm from treated males were motile and appeared normal in their behaviour in the female spermathecae. Dominant lethality induced by tepa was usually expressed during the embroyonic stages, but occasionally was delayed until the pupal stage.