The genetic structure of populations of an arctic-montane herb, Saxifraga hirculus (Saxifragaceae), was analysed by means of chloroplast restriction fragment-length polymorphism. Sampled populations were distributed across Europe and North America (Alaska and Colorado). There was no evidence for geographically structured genetically divergent lineages, and although no haplotypes were shared between North America and Europe, the haplotypes from different continents were intermixed on a minimum spanning tree. European populations were much more highly differentiated and had much lower levels of haplotype diversity than their Alaskan counterparts. Centres of haplotype diversity were concentrated in those Alaskan populations located outside the limits of the last (Wisconsin) glaciation, suggesting that they may have acted as refugia during the Pleistocene. It was not possible to identify putative migration routes or corresponding refugia in the European genepool. One British population, from the Pentland Hills, was genetically very distant from all the others, for reasons that are as yet unknown.