Detailed investigation of two areas on the Wirral Peninsula has revealed populations of Arion ater and A. lusitanicus in comparatively undisturbed habitats. These populations appear to be mutually exclusive. There are considerable differences in morph frequencies both between species and between populations of A. lusitanicus. A. lusitanicus appears frequently to mature early while still retaining some juvenile colour characters, and populations maintain a much higher density than do adjacent populations of A. ater. A. subfuscus is present but only with A. lusitanicus. Preliminary evidence is given that the colour morphs of A. lusitanicus are genetically determined. The significance of this type of distribution of closely related species is briefly discussed.