ABSTRACT. The sexual behaviour of male Glossina morsitans morsitans Westwood (Diptera: Glossinidae) towards decoys dosed with sex phero-mone (15,19,23-trimethylheptatriacontane) is described quantitatively from field observations. The flies responded with a standard behavioural pattern after landing. This involved: first, the perception of pheromone; second, orientation on the decoy and genitalia engagement; and third, a lengthy quiescence corresponding to natural copulation. The probability of a fly leaving a decoy decreased during the sequence and was least in the final ‘copulatory’ phase, when it remained constant under constant conditions. The initial, rapid rates at which flies left decoys were affected mostly by pheromone dose: the lower the dose, the higher the rate. The final, slow rate of leaving was unaffected by pheromone dose, being most affected by environmental stimuli, especially interference from other flies: at high fly densities, final rates of leaving were high. The results are discussed in the context of improving the potential for using pheromone and chemo-sterilant-dosed decoys for tsetse autosterilization.