Nyssa sylvaticaseedlings grown for a year under flooded conditions established endomycorrhizal associations withGlomus mosseae. Lowland ecotypes with endomycorrhizae showed a significant increase in biomass over non‐mycorrhizal controls; the greatest difference was in aboveground biomass. Upland ecotypes survived flooding poorly and established fewer endomycorrhizae than lowland ecotypes and did not show any enhanced growth over controls. In general, most endomycorrhizae were formed near main roots and significantly decreased in abundance with distance from the main axes of the root system. It is suggested that under flooded conditions internal oxygen transport may be limiting to mycorrhizae in the more distal roots.