The distribution ofArianta arbustorum, Cepaea nemoralisandC. hortensis, and the variations in the shell polymorphism of theCepaeaspecies have been studied in an area of low chalk downland and alluvial river valleys near Rickmansworth, Hertfordshire. The distribution ofA. arbustorumis restricted, the species being largely confined to the bottoms of river valleys, especially in hedgerows. Variations in dampness seem the most likely limiting factor.C. hortensisis widespread and found in many habitats, butC. nemoralisis largely confined to woods, and is especially common in beechwoods. The morph‐frequencies ofC. nemoralisandC. hortensisvary in such a way as to suggest the operation of climatic and visual selection. InC. hortensisthere are area effects for the presence or absence of pink five‐banded shells, and there is an overall linkage disequilibrium resulting in an excess of unbandeds in non‐yellow shells. Reasonable evidence for the operation of random factors in determining the morph‐frequencies of populations ofCepaeain the area has been found only for three very disturbed populations. It is suggested that the large number of selective forces known to act onCepaeapopulations in this area will account for the variation in morph‐frequencies in populations from the same area o