In Buenos Aires province, the river Samborombón, which is typical of the plain, was sampled monthly during 1986-1987 in
research carried out both on its physico-chemical characteristics and the communities present - plankton, phytobenthos, zoobenthos -. In this paper the structure and function of phytoplankton and benthic algal communities in the different sections of the river are compared. Both communities have definite limits as their components are not shared and show different strategies. Likewise, the environmental conditions along the river are variable, leading to differences in the planktonic and benthic algae at the locations studied.