Maple leaf disks were exposed in a softwater stream for two weeks and then aerated for 2 d in water to induce spore production by aquatic hyphomycetes. Sporulation was inhibited by the addition of yeast extract or leaf leachate (up to 2·5 g l−1) to the sporulation water. With leaf leachate, but not with yeast extract, this effect was neutralized by the addition of 50 mg l−1 of CaCl2. Sporulation increased at intermediate concentrations of several inorganic compounds (CaCl2; KH2PO4/K2HPO4; NaNO3; NaCl). It increased at low concentrations (1–3 g l−1) of an artificial seasalt mixture, was much reduced at 7·5 g l−1 and absent at 15 g l−1.