Overwintering of larvae of the Australian sheep blowfly, Lucilia cuprina (Wiedemann), was studied over 2 years in south-eastern Australia. There was a biphasic pattern of emergence of flies from larvae entering the ground during April. Some larvae deposited in April developed immediately and emerged in late autumn, whereas others entered a state of arrested development and resumed development the following spring. Overwintering mortality was generally high, but varied between years and between replicate cohorts deposited at the same time. Overwintering larvae resumed development in early to mid-September, coinciding with an increase in soil temperatures of 1.5°C over a 4-day period and soil temperatures remaining above 11°C for at least 7 consecutive days. Spring emergence of the first generation of flies after winter was synchronous from larvae deposited at different times. The first flies were detected on 2 October and 26 September in consecutive years and emergence continued for up to 50 days. Free-ranging flies were first caught in traps on 17 and 7 October in consecutive years, and numbers showed a large peak in late November and a smaller one in early March. No flies were caught from May to September. More detailed studies on the immature stages of L. cuprina, such as the conditions that induce and terminate arrested development of post-feeding larvae, and those that determine the survival of larvae and pupae in the soil, are needed. This knowledge could be used to refine control programs based on the treatment of sheep with long-acting insecticide in September or October ('early season treatment'), which aims to prevent propagation of fly numbers from the first generation of flies and thus reduce the prevalence of fly-strike on sheep farms.