Spatial and temporal changes in conidial concentration of species of aquatic hyphomycetes were studied in three southwestern French rivers (Adour, Nive, and Tech). The survey was conducted from source to mouth for a year by water filtration. Fifty-two species were identified.Alatospora acuminataandClavariopsis aquaticawere the most abundant species in all three rivers. Most other species made small contributions to the total amount of conidia, but species such asHeliscella stellataorLemonniera aquaticaoccasionally may be important in a particular river. The timing of seasonal peaks of conidia concentration of some species was studied. Species were classified into three groups according to the similarity or differences among rivers. Peak conidial concentration could appear at the same season on the three rivers (e.g.,Alatospora acuminata,Articulospora tetracladia,Heliscella stellata,Lemonniera aquatica,Lunulospora curvula) or in different seasons on the three rivers (e.g.,Pyricularia submersa,Tetrachaetum elegans). No peak was found forClavariopsis aquaticaorClavatospora longibrachiataon any river. Changes in conidial concentration along the three rivers were analysed. Patterns of individual species varied among rivers and seasons. During summer on the Tech River, conidial concentration ofHeliscella stellataincreased abruptly to the highest values of this study after having remained close to zero in the uppermost 20 km. This coincided with a drastic decrease in community diversity in these reaches. Both altitude and downstream distance influence conidial concentrations in a complex manner.Key words: Ingoldian fungi, conidia concentration, season, elevation.