In Uganda the herbivorous land snail, Limicolaria martensiana occurs in well-defined populations. Samples of snails from 49 populations reveal the existence of five distinct colour forms, the frequency of which varies markedly from population to population. There is a common streaked form that occurs in all populations and a series of pallid forms that may be present or absent. Late Pleistocene fossils from the Western Rift indicate that the polymorphism is at least 8000–10,000 years old. The polymorphism is not correlated with broad geographical components of the environment or with habitat. It is however correlated with population density, and in one population a change in density resulted in a change in the relative frequency of the colour forms. Human activities have probably contributed to the dispersal of the snails to new areas, and because of these the founder effect cannot be ruled out as contributing to the polymorphism.