This work investigated the occurrence of aquatic hyphomycetes in stream ecosystems in New Zealand. It provided evidence for the potential importance of detrital cycles in such ecosystems and of the role of aquatic hyphomycetes in the decay of leaf litter. Leaves from a representative range of plant species were enclosed in mesh bags in two undisturbed streams flowing through different forest types at different altitudes, and fungal population dynamics followed during leaf decay. A method was developed, using pure cultures of appropriate aquatic hyphomycetes, which enabled the mycelial content of leaf material (m mycelium/g) attributable to individual fungal species to be quickly and easily determined from the spore production measured from leaf samples. Antibiotics were used to quantify the relative fungal and bacterial contributions to microbial activity of leaf samples through the selective inhibition of respiration. Fungi accounted for about half of the total microbial respiration, and on average 80 % of this fungal respiration was attributable to aquatic hyphomycetes. Three major patterns were apparent from the data for aquatic hyphomycete population dynamics. 1) A successional pattern was identified among aquatic hyphomycetes, with the species falling into three groups on the basis of the order of their period of abundance. 2) Aquatic hyphomycete abundance varied among leaf types, with soft-rotting types (hangehange, young kanono) being poorly colonised compared to firmer, darker-rotting types (old kanono, kamahi, silver beech, tawa, kauri, and rimu). Nikau (palm) and ponga (fern) leaves, although not soft-rotting, were also poorly colonised by aquatic hyphomycetes, but bore a rich alternative mycoflora. 3) The occurrence of many aquatic hyphomycetes showed seasonal preferences and a distinct geographical distribution which followed that of higher plants. Both seasonal and geographical patterns were probably related to temperature. Laboratory evidence was obtained of the potential importance of oxygen in determining aquatic hyphomycete occurrence, with the dominant species in most New Zealand streams (Lunulospora cymbiformis) being extremely intolerant of low oxygen conditions. Two new species of aquatic hyphomycetes were isolated in the course of the work.