Emergence of overwintering tobacco budworm,Heliothis virescens(F.), was monitored in Arizona, and the corresponding thermal accumulations were determined. Nondiapausing pupae emerged after the accumulation of ca. 176 centigrade heat units (CHU) when a threshold temperature range of 12.8 to 30°C was used. An accumulation of ca. 173 CHU from 1 January (5-cm soil depth) was required for emergence of first moths from overwintering pupae. Heat unit accumulations were less rapid with air temperature data. These data suggest that the physiological processes associated with diapause are nearly complete by early January. Alternate hosts are expected to play an important role in the early season since most, if not all, spring emergence occurs before the availability of cotton as a host plant.