Corylus avellana L. (hazel, Betulaceae) is a long-lived, widespread shrub in Europe, having its northern range margin in Fennoscandia and a postglacial history involving range-expansion from refugial areas in southern Europe. In this study, we tested for a relationship between marginality and low within-population genetic diversity by assessing patterns of variation at 14 putatively neutral allozyme loci (comprising 43 putative alleles) within and between 40 natural populations of C. avellana along a north-south transect in Europe. Geographically marginal populations (central Sweden) showed lower levels of within-population diversity than populations in more central regions, as indicated by significant negative correlations between latitude and the percentage of polymorphic loci (r(S)=-0.47, P < 0.001), the average number of alleles per locus (r(S)=-0.65, P < 0.001), the expected heterozygosity (r(S)=-0.19, P < 0.05), and the proportion of distinguishable genotypes (r(S)=-0.56, P < 0.001). These patterns, combined with the unusually high between-population component of gene diversity (G(ST)=19.7%) and allelic richness (A(ST)=24%) in the marginal region, can be attributed to historical bottlenecks during the species' postglacial range-expansion, but may also reflect a history of genetic drift in the small, isolated populations occupying the marginal region. Information on the spatial distribution of genotypes provide further support for a role of vegetative reproduction (layering) in the structuring of genetic variation within populations.