This is the first account of the effects of drainage water from an abandoned coal mine on the seaweed community of the intertidal rocky shore in the British Isles. At St Monans, Scotland, along a 250 m transect, the brown climax species Fucus spiralis showed a low abundance in sampling quadrats (17.5%) and small plant size (3.5 cm), close to the outfall, compared to stations at increasing distance from the drainage source. Immediately below the outfall the fucoid community was replaced by a mixture of 12 species of freshwater, brackish and marine Chlorophyta, which did not occur, or only scarcely occurred, at any other transect station. At high tide, ferrous iron concentrations in the water were highest near the source (15.5 mg/l), while salinity was at its lowest (1.1 psu). Accordingly, iron tissue concentrations of apical segments of F. spiralis were also at their highest (12 mg Fe/g dry weight). Reproductive development of F. spiralis may have been inhibited close to the source.