The polymorphism of shell colour and banding in Cepaea nemoralis is the most vivid of any species of land snail in western Europe. Rules of colour variation in relation to degree of exposure to sunlight have been suggested by Pilsbry for North American snails in particular, with an indication that they are of general validity. A survey of shell colour and banding in west European snails in relation to habits and habitat using data from Germain confirms Pilsbry’s generalizations for whiteness and opacity in very exposed species, and dull brown colour and transparency in very secretive species or those in shady habitats. Certainly at the extreme of shading and probably at that of exposure, no polymorphism is normally visible; highly polymorphic species including C. nemoralis have intermediate habits, and occur in a considerable variety of vegetation, being often visible during the day but not exposed for weeks on end to extreme insolation. The polymorphism in Cepaea nemoralis is a marked example of a more general phenomenon, probably appropriate to its mode of life.