Fish assemblages and environmental variables were studied in 245 sites in small and mid-sized rivers of the Tatarstan Republic. The basic aim of this investigation was to model the habitat requirements of the 26 most common freshwater fish species at the eastern edge of Europe since the spatial variation of the assemblages of these fish species is little understood there. The impact of environmental factors on species distribution using both presence-absence and abundance data was analyzed with regression methods and ordination methods, which are widely used by European ichthyologists elsewhere. Ordination analysis demonstrated an upstream-downstream gradient of the fish assemblages. Regression analysis indicated that basic predictors of species preference were elevation and “size” of river. For the first time in Eastern Europe, optimum points and niche breadth for fish species along basic environmental gradients were estimated. Niche breadth (tolerance) was highly variable between species. Generally, estimated relationships between environmental factors and the probability of fish presence and abundance were consistent with known fish ecology.