Current evidence indicates that fungi in streams are essentially restricted to coarse particulate organic matter such as decomposing leaf litter and wood. As the size of organic particles decreases, the proportion of fungal biomass also decreases. In coarse particulate organic matter, fungal biomass can be substantial, in some streams exceeding 15% of the total detrital mass in relatively labile types of leaf litter. Aquatic hyphomycetes are the predominant fungi occurring in this habitat. In all direct comparisons between leaf-associated fungi and bacteria which have been carried out to date, fungal biomass greatly exceeded bacterial biomass, typically accounting for more than 95% of the total microbial biomass. Maximum growth rates of fungi have been found to vary from 0.02 to 0.2 day-1, indicating, in combination with the high fungal biomass in leaves, that a substantial leaf-associated production occuts in streams. Like biomass, fungal production also exceeded that of bacteria, in the two comparative studies published to date. A significant portion of this fungal production is eventually liberated as conidia. Up to 7.5 x 106 conidia, corresponding to about 5 mg, have been found to be released per g of leaf litter per day. However, considerable differences in leaf-associated sporulation and fungal biomass exist between different leaf species and streams. Correlational evidence suggests that the development of fungi in leaf litter is controlled by both the nutrient concentration in stream water and leaf quality, as determined by the concentration of refractory leaf constituents such as lignin. Fungal activity may in tum control the breakdown rate of leaf litter. The annual leaf-associated production of fungi determined in a headwater stream (34 g dry mass per m2 of stream bed) agrees well with the result of a rough calculation that is based on average leaf inputs to streams, leaf retention efficiencies at base flow and estimated fungal biomass in decomposing leaves. Both figures are also well in the range of estimates obtained for macroinvertebrate production in streams. Taken together, the presented evidence thus points to a central role for fungi in stream ecosystems. With the suite of methods now available, adequate consideration of these organisms in stream ecology should be facilitated in the future .