Emergence in the laboratory at 80°F. of apple maggot adults from pupae overwintering outdoors was compared at different times of the dormant period and with emergence from pupae stored at 37-40° for all or varying parts of the dormant period. Shorter incubation times were required for emergence when pupae from outdoors were placed in incubation in autumn, one to three months after larval maturity, than during winter or early spring. Exposure for at least six weeks to low temperatures, either outdoors or in the laboratory, at first increased the incubation time required for emergence. Later the required incubation times of insects overwintering outdoors or in the refrigerator decreased as the duration of exposure to low temperature (or duration of dormancy) increased. But no such decrease was evident in samples of pupae held at 37-40°F. for periods longer than six months. Comparison of emergence from pupae from outdoors with those stored at 37-40°F. when incubated in spring and early summer showed that outdoor temperatures before June 1, in 1959-61 at least, had no effect on the subsequent emergence from pupae overwintering outdoors. When incubation at 70°, 75°, 80°, 85° and 90°F. was compared, the times required for emergence decreased with increased incubation temperature up to 85°F. However incubation at 90°F. resulted in a decrease in the percentage of incubated insects that emerged as adults.