The distribution of the riparian (bank) and aquatic vegetation of the River Wye is described in relation to soil and water chemistry, climatic factors and, where appropriate, height above river level. 2. All the measured water quality factors (i.e. conductivity, turbidity, pH, K', CaP, Mgr, CO3-- HCOL- N, P and ash) increased downstream, with concomi- tant changes in aquatic vegetation from species characteristic of fast-flowing, oligotrophic waters to those of slower-flowing, more eutrophic sites. 4. The distribution of the riparian species showed a gradient of change along the river, except where a substrate of blanket peat rendered species composition very different from the other sites. The floristic similarities between pairs of sites decreased with increasing separation of sites along the river, suggesting a vege- tational continuum. 5. For three major areas of the river, the distribution of species up the banks is described in relation to river level in the summer of 1976. Tolerance of sub- mergence, alluvial deposition and erosion, and competition is thought to play a major role in determining each species' position on the bank. 6. A comparison of the riparian species of the River Wye and the Missouri, U.S.A., shows similarities at the familial and life-form level and selected aquatic species of the River Wye and the River Tweed show similarities of distribution with respect to altitude and water chemistry, indicating a biological affinity between the two river systems.