Laboratory studies of diapause induction in the horn fly Haematobia irritans irritans (L.) indicated that the incidence of diapause was unrelated to day length or the number of day-night cycles. Transfer experiments were conducted to estimate the beginning and end of diapause sensitivity and the required number of days for diapause induction. On average, sensitivity was between the ages 0.10 and 0.82 on a physiological age scale of immature development. The number of days required for 50% diapause was estimated as 19.1 days. A model was developed relating the proportion of insects in diapause to the number of days spent in the interval 0.10-0.82. Additional experiments revealed that diapause in the puparia was related to the mean temperatures experienced by immatures during the interval of sensitivity. An equation relating the proportion in diapause to temperature was also developed and compared with the required day number model. Both models provided similar estimates of the proportion of horn flies entering diapause.