We examined the changes in composition of fish assemblages for all seasons in the Rı́o de la Plata estuary (36°S, 56°W), and identified the parameters that control the main structuring of the estuarine community. Multivariate analysis (DCCA), performed on species biomass (tnm⁻²), indicated a spatial structuring of the estuarine community in three areas: inner, central and outer. Bottom salinity was the main factor associated with the significant differences in species composition. Examination of the estuary as a whole indicated a persistent spatial pattern, and suggested that temperature has the strongest influence on seasonal structure of fish assemblages. Both seasonal migrations (for example, of Trichiurus lepturus and Discopyge tschudii) and reproductive movements (for example, of Micropogonias furnieri, Mustelus schmitti and Cynoscion guatucupa), appear as important factors affecting the temporal structure of the estuarine community.