The aim of this work was to evaluate the potential of aquatic macrophytes as biological indicators of the environmental conditions of rivers in north-eastern Spain. To this end, twenty five river basins were studied to assess the species composition and abundance of macrophytes, and to examine their relation with different geographic, morphometric, mineralization, and trophic status parameters. Of the twenty six macrophyte species found, five appeared to be useful as biological indicators of the environmental conditions of the rivers. L. minor was a good indicator of high mineralization (i.e., conductivity and Cl-) and high trophic state (especially, N-NO2-). C. stagnalis was a good indicator of high P-PO43- concentrations. P. crispus, P. polygonifolius, and R. penicillatus seemed to be useful indicators of high N-NO3- concentration. We conclude that, in our study area, aquatic macrophytes can indeed be used as biological indicators of the environmental conditions of rivers.