In the plains of the Tohoku district, the adults of the Siberian cutworm, Euxoa sibirica BOISDUVAL, which presumably returned from alpine aestivation sites, reproduced mainly in mid-September. The eggs laid into loose upper soil underwent a hibernation diapause after they completed embryonic development. In a semi-outdoor condition their preoverwintering hatch was negligible. Incubation of the eggs at 18°C after being kept in an unheated room for different periods showed that the diapause was gradually eliminated towards winter. Exposure to a sub-zero temperature, especially for more than 70 days, was effective in elimination of diapause. Partial elimination of diapause was also possible under higher temperatures. When eggs chilled for more than 100 days were incubated at temperatures above 18°C under moist conditions, their hatching was unimodal and mainly occurred during the first three days of incubation. Under dry conditions, only part of the eggs hatched within the first three days, the remainder not hatching until immediately after receiving a moisture supply on the 6th or 11th day. From the results, the life-cycle sequence was estimated and its adaptation to the climate and the seasonal growth of larval food plants was discussed.