A significant increase in mortality was observed in Glossina morsitans morsitans maintained for approx. 45 days on rabbits immunized with tsetse or bovinetrypsin. Tsetse and bovine trypsin were observed to cross-react when tested with an immunodiffusion technique. A significant decrease in fecundity was observed in flies fed on bovine trypsin-immunized rabbits. Pupae collected from flies maintained on rabbits immunized with tsetse and bovine trypsin were significantly lighter. A significant number of flies maintained on rabbit immunized with thoracic muscles, gut bacteria and bovine trypsin had permanently laterally extended wings.