Different statistical techniques were applied in an attempt to relate the physical and chemical variables of river water to the distribution of aquatic plants. The results show a low frequency of occurrence of aquatic plants (only 13 species were found) distributed over 53 sampling sites. A few plants were found in river headwaters with pronounced slopes: these had turbulent, non-polluted waters with high oxygen concentration, but with high mineral content (minimum conductivity value 200 S/cm). Plants were more abundant and frequent in sites with high organic loads in sampling sites located downstream of a small dam. The range of eutrophic conditions or mineral concentration over which plants were found was different from other European rivers, suggesting that the possible indicative nature of the species may differ. The presence of aquatic plants is related to chloride, which favours plant growth, and ammonium, which has a negative effect. A characteristic of the rivers studied, especially in middle and downstream zones, is the high chloride concentration associated with high nutrient content and correlated with the occurrence of aquatic macrophytes. However, the concentration of ammonium, an ion which is derived mostly from wastes (pollution), is associated with the absence of plants. This may indicate that the ammonium concentration is toxic to aquatic plants. In this sense, the presence of aquatic plants indicates an absence of toxic effects from pollution (due to urban and industrial dumping). It is possible to relate the presence of aquatic plants to the conditions of eutrophy and alteration of the environment and, hence, to use these plants, to a limited degree, as indicators of the degree of contamination or degradation of the rivers of Biscay. The following indicative character for the listed species are proposed: Apium nodiflorum is a species characteristic of zones with high nutrient concentrations; Groenlandia densa, Zannichellia palustris and Alisma plantago-aquatica are all indicative of clean and more or less stagnant waters; the presence of Lemna minor is generally associated with water of greater depth, less current velocity and high nutrient content, generally places affected by human intervention; Veronica beccabunga and Potamogeton pectinatus are characteristic of back-flowing and mesoeutrophic streams. Nasturtium officinale