The habitat preferences, temporal dynamics in density and the life history of two species of mayflies, Metamonius anceps and Meridialaris chiloeensis, were investigated in a Patagonian mountain stream over a period of one year. Benthic samples were taken in a low order Nothofagus forested stream. M. anceps was univoltine with small nymphs predominating in winter and emergence occurring in summer. M. chiloeensis showed a bivoltine life cycle with a winter and summer generation, and emergence period in spring and summer. Both species were significantly more abundant in the low water period than in the high discharge season. Gravels supported significantly more individuals of M. chiloeensis than sand in pools and leaf packs. Temporal overlap of abundance between species was low. The existence of different life history strategies and the temporal shift of main generations and density peaks were critical to allow species coexistence.