Spatial and temporal changes in aquatic hyphomycetes communities were studied in three southwestern French rivers (Tech, Adour, and Nive). The survey was conducted from source to mouth for a year by water filtration. Changes in conidial concentration, species richness, and population diversity were analysed. Concentration of conidia of aquatic hyphomycetes generally increased in the downstream direction. However, these changes varied according to the river and the sampling date. Conidial concentration was higher during autumn than during spring and summer, but there was no difference between the beginning and the end of the leaf fall. Species richness was maximal during autumn and winter. It increased rapidly in downstream direction and reached a plateau. The diversity of conidial community of aquatic hyphomycetes generally remained constant regardless of the river sampled or the sampling date. It approached 75% of the theorical maximum possible diversity value, given the number of species and number of conidia sampled.Key words: Ingoldian fungi, community structure, season, altitude.