Diapausing prepupae of the sunflower moth,Homoeosoma electellum(Hulst), did not survive>7 consecutive days at −5°C in the laboratory. However, chilling diapausing prepupae in darkness at 0°C for up to 14 days resulted in significantly quicker pupal development and a greater survival than either chilling them for a longer period of time in darkness or holding them at a higher temperature or longer photophase. Prepupae survived winter field conditions at Bushland, Tex., to pupate successfully and produce emerging adults. Mean initial emergence for 1981–1983 occurred on Julian day 137, compared with a predicted mean initial emergence on Julian day 134 ± 12 based on 7 years of soil temperature data and a developmental threshold of 15.24°C. Computation of degree-day accumulations from soil temperature data from a depth of 10 cm from 51 locations throughout the Great Plains enabled development of a simple model for predicting mean initial emergence of the sunflower moth in areas where it survives. Data from independent field studies on sunflower moth overwintering closely fit the predictions of our model for the southern Great Plains.