In the district around Belleville, Ontario, the eggs of Mantis religiosa are deposited during the end of August and throughout September. Some embryonic growth occurs after the eggs are laid and before they enter diapause for the winter. Many of the eggs which have over-wintered, however, fail to continue their development so that the percentage of hatch in the following spring is often greatly reduced. This egg mortality varies from year to year. Records obtainrd between 1940 and 1946 by the dissection and incubation of current oothecae collected at the end of March have indicated mortalities ranging from 15.6 to 86.4 per cent. Such mortality has been generally attributed to low winter temperatures although there was also evidence that eggs could be exposed to a temperature as low as−20°C without injury.