June populations of Hexagenia naiads in Pool 19 of the Mississippi River were estimated at 3.6, 18.7, 6.7, 23.6 and 11.9 billion in 1959 to 1963, suggesting an alternate year cycle of abundance. Although population density varied in different sections of the river, little pattern was evident. Water depths from 0.5 to 6 m and bottom deposits from soft mud to soft muds with shells or plant debris had little consistent relation to population density. Sand and gravel areas had few or no Hexagenia. Naiads increased in average size in June. In late July after major emergences, there were few naiads collected, but small naiads increased in Aug. Flood may have shifted part of the population in 1960. Tendipes plumosus larvae were more abundant in 1961, when Hexagenia populations were lower, than in 1960. Usually H. limbata was the more abundant species (among naiads over 16 mm) in early June and in Aug. and H. bilineata, in late June and July. Sex ratios were usually near 50:50.