A study has been carried out of zinc enrichment in streams draining the Northern Pennine Orefield and the effects of this enrichment on the algae. The sources of the contamination are described and are attributed mainly to continued discharge from adits, and drainage from tailings heaps associated with the disused lead-zinc mines. Details of surveys performed on 10m reaches situated on 45 streams and an intensive survey of 30 reaches on one stream (Gillgill Burn, Nenthead, Cumbria) are given. Using factor analysis to evaluate the interrelationships of the inorganic chemical components, the weathering of carbonate minerals was indicated as being an important factor in controlling the ionic composition of the waters. With increasing levels of zinc, fewer species of algae were represented. Of those species that were present at the higher levels, the genus Hormidium was frequently evident. In particular, H, rivulare was widespread and often also abundant. A survey of 47 reaches with both high and low levels of zinc was carried out to establish the extent to which populations of Hormidium spp. taken from the former were in fact resistant to the zinc. It was demonstrated in the laboratory for H, rivulare that this resistance was largely, if not entirely, due to genetic adaptation. An assessment of the influence of environmental factors in the field on the toxicity of zinc to H, rivulare was made by comparing the results of laboratory bioassays with the detailed water chemistry of the sites from which populations were taken. These tentative results were compared with experimental studies in the laboratory. The important factors increasing toxicity are Cd and a rise in pH. Factors leading to a decrease in toxicity are Mg, Ca, and PO(_4)-P
