Donax rugosus is a common member of the intertidal fauna on wave-beaten sandy beaches in West Africa. In Sierra Leone, the animals breed during December and the shell grows about 3 mm a year to 24 mm after seven years. All populations studied are polymorphic with respect to shell colour, more so in Sengal than in Sierra Leone. Both the relative frequency of the white form and the total number of forms present in the same population increases with population density. There is little indication of significant differences in the relative frequency of colour forms in different year groups from the same population. The polymorphism is therefore assumed to be fairly stable at least over the short term.