This paper presents a taxonomic and ecological study of Bacillariophyceae in the Paraná River (Argentina). Qualitative and quantitative monthly samples were taken in eight sampling stations, between 1976 and 1984. Cell counts were performed using the Utermöhl method (1958). Major physical and chemical characteristics of the stretch of river are described. Bacillariophyceae density is compared with total phytoplankton and major environmental variables (hydrometric level, transparency, temperature, pH, conductivity, dissolved oxygen and nutrients) in each sampling site. We present 145 taxa classified in 35 genera: 11 Centrales and 24 Pennales. Centrales were represented by 20 taxa (11%) and Aulacoseira granulata and its varieties numerically predominated in the phytoplankton in most of the studies. Pennales appeared in a lower density but in a higher specific richness, totaling 125 taxa (89%). All registered taxa are illustrated and both their geographic distribution and ecological preferences are provided.