The lichen vegetation of forty-five acid watercourses in England is described and related to ecological variables. The zonation scheme adopted for chalk and limestone streams that involves four overlapping bands of lichen vegetation related to length of submergence is also appropriate for this habitat. The flora involves around 90 species, a number of which are rarely recorded specialists. Evidence is presented that substratum chemistry is a more powerful ecological factor than water chemistry. An attempt has been made to relate the lichen flora to homogeneous stream segments from the headwaters down to the alluvial lowlands. An investigation into the effects of water quality showed that both suspended solids (silting) and eutrophication are highly deleterious to the lichen flora; the submerged and fluvial mesic zones are the first to be affected. The zonation across streams in England is compared with those reported from Germany and found to be closely similar.