Shell shape and colour variation is common in littoral molluscs. As far as shape is concerned, exposure to wave action acts as a strong agent of natural selection, but only speculation seems to have been directed towards elucidating the determinants of colour variation. In the dog-whelk (Nucella lapillus (L.)) diet may play some part in producing the pigment for colour. However white and banded forms are often found in otherwise coloured colonies. This study describes the classification for colour and banding of 83,524 whelks from 444 areas around the British Isles and west Atlantic coast. Colour and banding were significantly correlated r=0.45±0.05). Because of the possible involvement of diet in shell colour, attention has been confined to banded forms. No obvious pattern in distribution could be discerned. Possible factors affecting the occurrence and incidence of banding are: climate; geology; exposure to wave action; predation; intra- or interspecific competition; cryptic genetical factors; or intraspecific recognition. The only one which was definitely associated with banding frequency was exposure (r= 0.33±0.05), but there were so many exceptions to the correlation (e.g. highly exposed populations which were entirely unhanded), that this is unlikely to be causal. The most probable explanation seems to be the local spread of mutant forms (probably linked to fitness traits), which have not attained equilibrium frequencies–even though N. lapillus has apparently occurred in large colonies round much of the British coast since the end of the Pleistocene.