ABSTRACTWater from the upper San Marcos River in South Central Texas was sampled for amphibious hyphomycetes monthly for one year. The headwaters were chosen because temperature, pH, and electrical conductivity remain constant. Analysis of filtered water, foam, and decaying leaves revealed 13 species. Populations of these organisms vary seasonally; the greatest spore density coincides with periods of greatest leaf abscission.Lunulospora, Tetracladium, Triscelophorus, and aClavatospora-like species are most abundant. Taxa present are restricted to those species able to sporulate at 22 C, and seasonal fluctuations of these fungi are more dependent on substrate availability than on the physical conditions of the river. Direct observation indicates thatLunulospora curvulaandTetracladium marchalianumare ubiquitous on leaf substrates.