Populations of the gastropods Littorina rudis and L. arcana on the rocky shore at Westdale, Pembroke, South Wales, have been investigated. Foot area has been related to shell length. It is shown that there are differences in the areas of the expanded foot of different groups of snails: those from more exposed sites have a relatively larger foot. Littorina arcana achieves the greatest foot area of all, and it extends on to the most exposed rocks investigated, where L. rudis is absent. It is suggested that this may be important in ecological separation of the two species. It is noted that there is still uncertainty over the taxonomic identity of what is referred to here as ‘Littorina rudis’, and further work is in progress.