Experiments were carried out to determine the effects of the insect growth regulators (IGRs), diflubenzuron and two of its analogues, on adult female Glossina morsitans morsitans Westw. The most effective method of dosing was by topical application, and less pronounced effects followed tarsal contact with treated surfaces and contact during mating with treated males. The compounds had no effect on female length of life or the number of offspring produced, but following topical application at 0·5 μg/female or above, most offspring produced throughout reproductive life (more than 100 days) failed to pupariate. Little difference was detected between the effects of the three IGRs tested. Following the application of diflubenzuron to oviparous insects, egg hatch is inhibited; it is suggested that in the larviparous tsetse fly such compounds act by inhibiting the biosynthesis of chitin at the time of pupariation. IGRs show promise as potential agents for the control of Glossina in the field, and the quantities required would probably not exceed the quantities of insecticides used in current operations against tsetse flies.