Abstract. Abstract: Aim This study aimed to understand the seasonal variation of epilithic diatoms in the Gravataí River regarding their composition and density along the river, as well as the respective relationships with local limnological variables. Methods The diatoms were sampled quarterly using a EDS sampler exposed for four weeks from September 2000 to August 2002. An area of 75 cm2 was scraped off the upper surface of three submerged stones totalizing a composed sample. An aliquot of 1 ml of oxidized material was mounted on a slide for the quantitative and qualitative analysis. Results A spacial organic contamination gradient was observed in the Gravataí river due the decrease of dissolved oxygen, and increase of BOD5, total nitrogen and ortho-phosphate from the upper to the lower course and the predominance of eutrophic conditions during periods of higher temperature (spring and summer). The epilithic diatoms presented, in the upper stretch, a seasonal pattern in abundance with the highest densities during the fall and winter, the density enhance was associated to mild and low temperatures, lowest turbidity and nutrients availability. Bacillariaceae was dominant in the spring, Eunotiaceae and Bacillariaceae in the summer and fall and Gomphonemathaceae was strongly dominant followed by Eunotiaceae in winter. In the lower course Bacillariaceae was dominant in all seasons except on fall 2001. These changes were determined by the substitutions in the occurrence and relative abundance of some species as Achnanthidium minutissimum, Cocconeis placentula, Eunotia bilunaris, Frustulia saxonica, Gomphonema parvulum, Nitzchia palea and Sellaphora seminulum. Conclusions The seasonality of the diatoms was well evidenced in upper stretch of the river, therefore in the lower course with greater anthropogenic impact, fewer changes occurs in the number of species present and/or of substitution of some species by others, showing that the seasonality of the diatoms can be affected by organic contamination and eutrophication.