Changes in conidial numbers of aquatic hyphomycetes in water passing through a concrete pipe were investigated by water filtration. Retention in the pipe was low and concentrations of conidia of Alatospora acuminata, Anguillospora longissima, Clavariopsis aquatica, Clavatospora longibrachiata, Heliscella stellata, Lemonniera terrestris, Lunulospora curvula, Pyricularia submersa, Tetrachaetum elegans and Tetracladium marchalianum were unchanged after 1·8 km drift. Only one species, Articulospora tetracladia, showed significant decrease, resulting in part from fragmentation of the conidia. These results suggest that studies of communities of aquatic hyphomycetes in natural environments by sampling conidia by water filtration need to account for differences in stream retention as well as species specific differences in conidial fragmentation.