The lichen flora of 40 lakes in England, Scotland and Wales is described and related to ecological variables. A zonation scheme consisting of four overlapping bands of lichen vegetation related to height above water level has been adopted. Due to the lack of scour, and the influence of wave action, they do not correspond exactly to the zones identified beside streams, so have been named ‘splash zones’. About a hundred species are involved, a number being rarely recorded aquatic specialists. The flora of all zones is strongly influenced by the alkalinity and pH of the water which have provided a framework for arranging the communities. Lichens are reliable indicators of the sequence: acid, very soft, soft and medium hard water bodies. An investigation into the effect of man-induced eutrophication was undertaken. Other factors of ecological importance are shade and humidity. The conservation of the lake habitat is discussed and a comparison made between streamside and lakeside communities.