Laboratory studies of diapause induction in the horn fly,Haematobia irritans irritans(L.), revealed that maternal photoperiod had no influence on the time of emergence of nondiapausing or diapausing horn flies. Larval rearing temperature affected emergence time of nondiapausing horn flies, but had little effect on emergence time of diapausing horn flies. Horn flies began emerging from diapause at 25°C after 60 d exposure to temperatures below 20°C. Emergence from diapause required<60 d at 25°C for completion. Diapause occurred in nearly 100% of insects reared at less than 16°C regardless of photoperiod to which the parents were exposed. No diapause occurred in insects reared at 25°C, regardless of whether the parents were exposed to 8, 12, or 16 h light. Intermediate levels of diapause occurred in insects reared at 20°C, but this showed no relationship to maternal photoperiod. Overall, larval rearing temperatures accounted for 89.2% of the variation in the incidence of diapause, while maternal photoperiod alone accounted for only 0.1%.