The annual diapause phenology, larval development, and adult emergence of tobacco budworm,Heliothis virescens(F.), was studied under insectary conditions at Phoenix, Ariz., from August 1986 to September 1987. The percentage of pupae entering fall diapause peaked at approximately 97% on 16 November 1986 and remained high (near 80%) through February 1987. The percentage of pupae entering summer diapause reached a maximum of 51.7% on 19 July as mean daily temperatures increased to 30°C. Moths emerging from nondiapausing and diapausing pupae contributed to major moth peaks in the fall (23 August to 1 December) and spring (1 April to 15 May) months. Very little moth emergence was observed from 15 December to 1 April. Larvae required an average of 10–88 d to complete development, depending on annual temperature and photophase fluctuations. Larval mortality (range, 1–93%) also was highly correlated with larval developmental time. Pupal developmental time varied greatly among cohorts: nondiapause, 10–45 d; fall diapause, 40–190 d; and summer diapause, 28–105 d. Pupal mortality for fall diapause (43.5%) also was significantly higher than that of other pupae. Lower and upper temperature thresholds for larval and pupal development, taken from the literature, were modified using coefficient of variation analyses of degree-day accumulations. Based on the coefficient of variation method, lower and upper thresholds for larvae, nondiapausing, fall-diapausing, and summer-diapausing pupae were 11.25–32.0, 12.8–34.0, 12.8–32.0, and 12.8–30.0°C, respectively. Use of this information to develop a season-long model ofH. virescensphenology is discussed.