The importance of maximum rates of insemination in female tsetse is emphasized in relation to their low rate of reproduction and to the possibility that density dependent regulation of tsetse populations may not be as rigorous as in many oviparous insects. Observations on the mating behaviour of tsetse are described and its regulation by sex pheromones which serve to identify females of the species is discussed. Laboratory and field experiments have shown that the sexual activity of G. m. morsitans males can be manipulated by presentation of synthetic sex pheromone on suitable decoys in the laboratory and in the field. Males that respond to decoys in the field are in the same nutritional state as others which are attracted to the models upon which the decoys are placed and are presumably seeking food. There is no reason to believe that sexual activity is restricted to a particular section of the population, and given a choice, it is proposed that mating is preferable to feeding in terms of gene survival for the individual male. The durations of copulatory responses are dose dependent and there is some evidence that they are inversely proportional to the frequency of encounters with decoys and hence to the density of flies. Techniques for the reduction of tsetse populations using sex pheromones in conjunction with suitable decoys and chemosterilants are being developed. The availability of pheromones in large quantities will permit their testing as sexual confusants among males and may lead to interspecific matings where more than one species occupies the same habitat.