Analysis of PCR/RFLP variation in 57 Nordic populations of the herb Silene dioica, revealed 13 composite chloroplast DNA (cpDNA) haplotypes. The geographic distribution of the haplotypes suggests that the postglacial colonization of Fennoscandia by S. dioica may have involved immigration of populations from two main directions. The commonest cpDNA haplotype dominates in populations throughout most of Finland and northern and central Sweden, but is absent from southern Sweden. The distribution of this haplotype is interpreted in terms of immigration from an eastern or northern direction. In contrast, eight haplotypes that are absent from northern Fennoscandia are represented in populations in southern Sweden and in Denmark, suggesting colonization by populations derived from one or several refugial areas further to the south in Europe. The overall NE-SW pattern of cpDNA haplotype variation is similar to, but less diffuse than, the pattern revealed by allozyme markers.