During 1985–1986, twelve rivers and streams belonging to the Delta Sub-basin (Rio de la Plata estuary Basin, Argentina) were sampled for microfauna associated with fifteen species of aquatic macrophytes. A total of 171 species were determined. Ciliates and rotifers were most abundant. Dissimilarities in the colonization of the macrophytes were demonstrated. Water level and current influenced the periphyton community and contributed to the differences observed.