> Abstract Twenty-seven species of tropical and subtropical fungi isolated from freshwater were examined for evidence of interspecific interactions, which are important in determining the ecological roles of fungi. Evidence for interspecific interactions was examined by inoculating paired fungi 25 mm apart on the surface of agar plates. The antagonistic activities were different among different isolates and even between isolates of the same species, for example, Ophioceras dolichostomum isolated from different origins. Pseudohalonectria longirostrum and Kirschsteiniothelia elaterascus, which produced pigment in culture, were strongly inhibitory species. Several aquatic hyphomycetes seems to be less competitive and less likely to produce antagonistic substances. Competitive abilities were also influenced by the range of enzymes that a fungus produced. For example, Verticillium sp. and Diaporthe sp., which produced only one or two kinds of enzyme, were found to exhibit weak competitive abilities and were easily replaced. The results of competition experiments also showed that slow-extending fungi (e.g., Pseudohalonectria longirostrum and Kirschsteiniothelia elaterascus) were more competitive than early fast-extending fungi (e.g., Ophioceras dolichostomum and Nectria haematococca).http://link.springer-ny.com/link/service/journals/00248/bibs/37n4p257.html