The genetic population structure of Polyommatus coridon (Poda, 1761) (Chalk-hill blue) was studied by means of allozyme electrophoresis in north-eastern Germany, the Czech Republic, Slovakia and Hungary. All analysed parameters showed high genetic diversity within populations (number of alleles: 2.61; observed and expected heterozygosity: 18.6% and 19.7%, respectively; percentage of polymorphic loci: total: 73.6%, on 95% level: 56.1%), whereas genetic differentiation between populations was comparatively low (F(ST) = 0.028 +/- 0.005 s.d.). Hierarchical variance analysis revealed significant structuring among five regional population clusters. A significant isolation-by-distance structure exists (r = 0.39; P < 0.05). The mean number of alleles per locus declined significantly from south to north and showed a strong correlation with the geographical latitude (r = -0.88, P < 0.0001). We suggest that this reflects the loss of alleles during the postglacial colonisation of eastern Central Europe from an ice-age refugium in the Balkans. A possible scenario for the postglacial expansion process in eastern Central Europe is discussed using these data: coming from the north-western part of the Balkans, P. coridon may have reached the western tip of Hungary, and consecutively colonised eastern Central Europe using two alternative expansion routes.