ABSTRACTIn vitrohyphal interactions of 25 species of leaf- and/or wood-inhabiting aquatic hyphomycetes and ascomycetes were characterized. In 96.9% of the interactions, growth of one or both of the members of an interacting pair was inhibited to some degree; stimulation of growth did not occur. Fungi could be distributed along gradients according to their ability to inhibit or resist inhibition. Generally, wood-inhabiting species were more inhibitory and resistant to inhibition than leaf-inhabiting species. When species rankings of ability to inhibit and resist inhibition were summed to give an interaction index, wood-inhabiting species ranked above leaf-inhabiting species. Early colonists on wood were exceptions to this pattern. Eight species produced zones of inhibition suggesting that they produce diffusible antifungal substances.