In the Canadian Rocky Mountain National Parks, the lodgepole needle miner has a two-year life cycle, so that larvae of each generation are exposed to two successive winters. Although there is no doubt that mortality of these over-wintering larvae is directly attributable to a period of low temperature, there has been some doubt as to whether the bulk of the mortality occurs during the coldest part of the winter, or whether it occurs during periods of low temperature during the late autumn or early spring. Although spring and autumn mortality undoubtedly take place, estimation of the relative importance of mortality during the various seasons must await successive sampling throughout the three seasons. Nevertheless, in the absence of such data, we wish to present evidence that points to the major importance of the weather of the coldest month of the winter to not only the amount but also the vertical distribution of larval mortality.