A population budget was constructed for Chaoborus flavicans in Lake Lenore, Washington, during the 1985 spring emergence to estimate the proportion of the population consumed by cutthroat trout. Weekly sampling of the population indicated that all individuals were competent to pupate and emerge and that cutthroat trout preyed heavily on the pupal stage. Field measurements of life-stage transformation rates and rates of fish predation were incorporated into a daily simulation model which indicated that cutthroat trout consumed 33% of all individuals during the spring emergence period. Length of the pupal period shortened as water temperature increased. This decrease in pupal period reduced the probability that individual pupae would be consumed by trout later in the emergence period. The simulation model was used to examine the effect of increased temperature on developmental processes. When lake temperatures were increased 2° and 5°C above measured values, a greater proportion of fourth instar larvae entering an emergence period survived to the adult stage. Consideration of developmental processes may help explain intra-annual variation in recruitment success of C. flavicans in Lake Lenore.