SummaryGeographic variation of six loci encoding enzymes (Est-F, Lap-2, Mdh-1, 6pgd, Pgi, and To-2) and of four loci encoding shell characters (C, B, S, and U) in Cepaea nemoralis was examined in the western Berkshire Downs, where there is an extensive area effect for the midband gene, U3.Significant pairwise associations of geographic variation were found for all combinations of the U, S, Est-F, and 6pgd loci. At each of these loci, an allele which is rare outside the midband area reaches moderate to high frequencies within that area. Significant associations of C with U, S, and Est-F, and of S with Mdh-1 also were found. All pairwise associations reflect the area effect.Multilocus geographic associations were examined by Varimax factor analysis. The midband area effect is well represented by the second Varimax axis, accounting for 20 per cent of the allelic frequency variation over all 10 loci. Variation at the C, U, S, Est-F, and 6pgd loci is correlated with this axis. The first Varimax axis represents structure within the midband area, accounting for 22 per cent of the allelic frequency variation, and with which several loci are correlated.That several loci are associated with the midband area effect indicates that the area effect reflects substantial genetic differentiation. Whatever the cause of the area effect is, it has acted pervasively.