The effects of iron and manganese on the growth and reproduction of three aquatic hyphomycetes, Alatospora acuminata, Tetrachaetum elegans and Articulospora tetracladia, were assessed in the laboratory. Previous studies had indicated that T. elegans and A. acuminata appeared not to colonize leaf material below a discharge from an abandoned coal mine, whereas the distribution of A. tetracladia was not affected by this effluent. The principal contaminants of this effluent were iron and manganese. Neither iron nor manganese had inhibitory effects on the growth of the three species investigated. Indeed, mycelial extension rates of A. tetracladia were stimulated by the presence of iron. Reproduction was however, affected, with the sporulation of both T. elegans and A. acuminata being significantly reduced by exposure to either iron or manganese. By contrast, no significant effect on sporulation of A. tetracladia was observed. There was no significant effect of either metal on conidial germination of the three species. The apparent field distribution of fungi was related to their sensitivities to metals in the laboratory. Those species which appeared to be excluded by the discharge produced few or no conidia when exposed to metals at concentrations equivalent to those recorded at the contaminated site. Since conidia were used to detect aquatic hyphomycetes, it is unclear whether A. acuminata and T. elegans were indeed absent below the discharge or were present as mycelium which, being unable to sporulate, cannot be identified.