A field survey of British river sites gave rise to the hypothesis that water velocity and alkalinity are two important factors determining the distribution of Batrachian Ranunculus species. (DCA axis 1: alkalinity R=.4, p<=0.01, DCA 2: discharge R=-0.6, p<=0.001). The effects of a third potential stress factor (reduced light availability) were also investigated in a field experiment. A number of plots were established on a side-arm of the River Frome, half of which were shaded. Ranunculus had significantly lower cover in t he shaded plots, and this effect increased during the growing season.