Bucculatrix canadensisella Chambers, an important defoliator of birch trees in Canada, overwinters as a cocoon in forest-floor debris. Cocoons obtained from the field over a 3-year period showed that high mortality attributable to climatic conditions could occur during this stage. Field and laboratory experiments indicated that cocoons were quite resistant to various conditions of moisture and to cold in the fall and through the winter, but that they were highly susceptible to dry conditions at the time of morphogenesis in the spring. Cool temperatures retarded adult emergence; in nature, this would expose cocoons to a prolonged period of predation. It is concluded that weather conditions during a critical period of a few weeks in late spring could greatly affect cocoon survival of this insect. Warm, moist weather would offer optimum conditions while cool and (or) dry weather would result in high mortality.