The effects of cold period duration and simulated rain on adult apple maggot,Rhagoletis pomonellaWalsh, emergence were investigated under laboratory conditions. Pupae were exposed to cold periods ranging from 9 to 191 d. There was no adult fly emergence from pupae exposed to ≤23 d of cold. The degree-day (6.5°C threshold) requirements for first and 50% emergence increased for pupae exposed to cold periods ranging from 37 to 65 d; however, flies exposed to longer cold periods (79–191 d) required fewer heat units for emergence. The effect of simulated rain (2 cm) on adult emergence was tested at various degree-day accumulations (677–1,065) during pupal development. Pupae exposed to rain at any of the degree-day accumulations tested required significantly fewer heat units for first and 50% emergence, compared with pupae not exposed to rain.