Concentrations of potassium nitrate, potassium phosphate, or calcium chloride were varied in stream-simulating microcosms containing leaf discs colonized by the aquatic hyphomycetes Anguillospora filiformis or Lunulospora curvula. Cumulative conidium production of the fungi determined from rates of sporulation at 2 d intervals was stimulated by increasing concentrations of both potassium nitrate and potassium phosphate but not by calcium chloride. Leaf weight loss was also stimulated by increasing concentrations of potassium nitrate and potassium phosphate. The amount of fungal biomass (determined as ATP concentrations) associated with leaf discs increased significantly only in the treatments in which A. filiformis received increasing concentrations of potassium nitrate. These results indicate that as aquatic hyphomycetes grow on leaf litter, they can obtain at least a portion of their inorganic nutrition from the water flowing over the leaves. They also suggest that sporulation is more sensitive to changes in nutrient concentrations than growth.