In the 1930s Cyril Diver sampled and scored populations of the polymorphic snail Cepaea nemoralis on the South Haven Peninsula, Dorset. His recording methods make it possible to be confident about the absence of the species from parts of the peninsula, which has acid soils and changing habitats, and is generally unfavourable for the species. A resurvey in 1999 revealed a pattern of retreats and extensions of distribution related to local environmental change. Where populations have persisted over the period, the pattern of morph-frequency distributions has remained the same; there is equivocal evidence for a small increase in the frequency of midbanded shells. While most wetland populations have disappeared, new populations have become established along a roadside, and on newly-stabilized foredune ridges. Morph-frequencies in these new populations relate to those of the nearest established populations, and are sometimes monomorphic. The results are discussed in relation to the history of the peninsula, and to the role of migration in determining observed morph-frequency distributions.