Winter moth hatching begins daily at approximately 0645 hours A.S.T. both in the field during the normal hatching season and in the laboratory during the winter. This diurnal pattern is not related to temperature or light and appears to be intrinsic.The seasonal rate of hatching in the spring was related to accumulated heat. Eggs first exposed to cold December temperatures then stored at 35°F (1.7 °C) until early spring, were incubated at a series of controlled temperatures. The theoretical threshold temperature of development was determined as 41°F (5 °C). Accumulated degree-days based on this threshold did not give the best estimate of the time of hatching in the field where temperatures, because they were recorded in Stevenson’s screens (meteorological cabinets), were generally lower than those at the egg sites. The best estimate was obtained by using a lower base temperature of 39°F (3.9 °C) which compensated for differences in temperature between Stevenson screens and egg sites. Mean hatch in the field occurred when accumulated degree-days reached 292.A predictive curve is presented which predicted the date of mid-hatch of the winter moth to within 1 to 2 days over a 9-year period.