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Survival patterns of cohorts are described during a population cycle of the vole Microtus townsendii near Vancouver, British Columbia, Canada. A two–year live–trapping study on both enclosed and unfenced populations showed that cohorts during the increase phase of growth lived longest and had the best survival. Smaller voles in the peak density spring cohort had poor survival, but survival increased during the peak density summer. Survival of cohorts in the decline phase breeding season was very poor. The suggestions are made that changes in spacing behaviour may cause changes in cohort survival and that the causes of rapid changes in survival need to be determined.