This investigation is the first intensive study of the mayfly fauna of a North American lake in which the primary pupose was to determine ths local distributional pattern of the nymphs together with the environmental factors responsible for it. It was found that the physical character of the bottom, light intensity, and wave action are the principal factors which interact to determine the distributional patterns. The positive thigmotactic response of certain species of nymphs is also an important influence in distribution and accounts for the absence of nymphs from habitats lacking physical support. Temperature and dissolved oxygen have little, if any, effect on nymphal distribution within lakes. However, Hexagenia nymphs are limited by oxygen to that portion of the mud bottom above the hypolimnion. Low mayfly productivity in Douglas Lake arises from the fact that only about 15 per cent of the toal bottom area is suitable for mayfly populations.