We investigated the relationships between water chemistry and the occurrence, distribution, physiology, and morphology of fish faunas . We examined 34 species (ca . 10% of the Argentinean freshwater fish fauna) from 120 localities (5 areas) situated between 26°15' S (Trancas, 'lucuman) and 38°30' S (Sierra de la Ventana, Buenos Aires) . Fourteen chemical features are described by : conductivity, total dissolved solids, temperature, pH, CO3 2 , CO3H -, Cl -, SO4 -, Cat+, K+ , Mgt+, Na+, Mg/Ca, Mg+Ca/Na+K . Three Basic Data Matrices considering the mean, maximum and minimum values of each variable for each fish species were used in a Cluster and Principal Component Analysis . Groups of species clustered in similar ways to particular water chemistries . Similarity was the common occurrence of species in a defined area and preference for a common range of the factors considered . Groups of species so defined showed patterns of distribution related to climate, environment, trophic state and hydrographic complexity. Each cluster included some eurytopic species which appeared together at extreme chemical and geographic characteristics . Twenty four species had ranges of tolerance for the 14 variables and evidence of a grouping according to these ranges. Eighteen species which occurred at maximum or minimum absolute values for more than one factor were ordered along an eurytopy -stenotopy axis. We support the statement that species with a larger tolerance range for most factors have a higher probability of being widely distributed . Astyanax fasciatus and A . bimaculatus tolerated the highest number of maximum and minimum values, followed by Jenynsia l. lineata, A. eigenmanniorum and Trichomycterus corduvensis . Groups of species based on chemical factors showed differences in the relative number of basic morphological types .