The purpose of this study was to establish the structure of the benthic insect community in 2 bays of the Winnipeg River. Sixteen permanent sampling stations were established in the river and sampled routinely. Another aspect of this study was to examine the level of radioactivity in the various components of the insect community. The insects were digested in concentrated nitric acid, evaporated on counting discs, and then counted for total beta activity. The total beta activity in the insects was compared to that in the river water in terms of a concentration factor. The beetle Hydroporus sp. had the largest concentration factor (66.09) of all the insects examined. A total of 36 types of organisms were collected in 342 quantitative samples in 1966 and 1967. The largest percentage of the total number of organisms were Hexagenia spp. (75 and 16) Chironomidae (9 and 23) and tubificids (22). The possibility of these organisms acting as indicators of pollution or of changing environmental conditions and changing levels of radioactivity was also considered. Since environmental factors determine distribution, numbers and species of organisms, this study included a consideration of oxygen, temperature, turbidity, velocity, and bottom type in relation to the various organisms. As a part of the Whiteshell Nuclear Research Establishment preoperational survey, this study will serve as baseline information for the continuing routine monitoring of the aquatic environment.