The survivorship of each instar of three cohorts of Leptodiaptomus minutus from Bluff Lake, Nova Scotia, is presented. High naupliar mortality but low copepodite mortality occurred. The previously reported values for the predation rate and size selection of prey by omnivorous zooplankton are compared with the observed mortality rates for the different sizes of instars. We estimate that predation by the omnivorous zooplankton could account for most of the naupliar mortality. Our analysis supports Dodson's hypothesis that size-selective predation by zooplankton can be a significant factor determining the proportion of a cohort that reaches maturity.