AbstractA study was made of morphological variation inRhynchostegium riparioidesfrom 105 sites in 71 different streams and rivers; 15 gametophyte characters were scored for each population. Nineteen water–chemistry variables were also measured at each site. Variation was observed in the size and robustness of the plants, dimensions and shape of the leaves, the degree of denticulation and relative length of the nerve. Cluster analysis revealed broad morphological groups, with low internal similarity. These results do not support the recognition of subspecific taxa. Correlations with environmental variables showed that plants were significantly less robust, smaller-leaved and with weaker denticulation when collected from nutrient-rich water. In view of the wide morphological range shown by this species and its importance for monitoring heavy metals, a comparison with other widespread aquatic mosses is given.