Data from 8 consecutive years of monthly sweep-net sampling in the littoral zone of Bays Mountain Lake are combined to show the life history of a "typical"generation of T. cynosura at this location (36° CN latitude, 550 m elevation). Population size peaks in mid-July, but declines by 60% before mid-August (mortality attributable to predation by both large dragonflies and sunfish). It then declines gradually through December, more steeply through the winter, and then gradually again until emergence in May. — Size-frequency distributions, and dry mass determinations for certain larvae, suggest that about 50% of the individuals in the penultimate instar in September are semivoltine; the others are fast-growing young-of-the-year entering the penultimate instar at a smaller size than members of the senior year-class. This size differential is maintained when larvae enter the final instar in October, and it appears that the smaller (junior year-class) individuals suffer more mortality during the winter, so that only about 25% of the emerging imagoes are univoltine.