Population dynamics of the mayflies, Hexagenia limbata (Serville) and Hexagenia bilineata (Say), were studied in Lewis and Clark Lake from 1962 to 1969. Nymphs that hatched in June and July emerged as adults in 13-14 months (titled 1-year cycle) and nymphs hatched in late August or September emerged in 22 months (titled 2-year cycle). Although an estimated 79% of the newborn nymphs begin life in the 2-year cycle group, only 53% of the emergent adults were in this group because of mortality. Standing crop was highest in the shore area, although a major migration away from the shore occurred in the spring. This migration was most intense when the population density exceeded 100/m2. Biomass ranged from 260 to 1070 mg/m2 dry weight over the study period. Instantaneous emergence rates were higher than predatory mortality rates but were only operative from about 1 July to 15 September.Production was estimated by two methods from extensive sampling in 1964 on a 1256 hectare section of the reservoir. The growth method yielded 1.50 gm/m2 dry weight and the Hamilton method, 1.67 gm/m2. The turnover ratio of 2.81 calculated from these data multiplied by the standing crop over the rest of the reservoir resulted in an average annual production of 15.6 kg/hectare for the reservoir.The Hamilton method was applied to less extensive collections from 1964 to 1969. Maximum production occurred in 1966 at 2.41 gm/m2.