The phylogeography and demographic history of two closely related species of the red wood ant (Formica pratensis and F. lugubris) were examined across Eurasia. The phylogeny based on a 1.5-kilobase mitochondrial DNA fragment, including the cytochrome b gene and part of the ND6 gene, showed one phylogeographical division in F. pratensis. This division (0.7% of nucleotide divergence) suggests postglacial colonization of western Europe and of a wide area ranging from Sweden on the west to Lake Baikal on the east from separate forest refugia. In two localities, mitochondrial DNA has been transferred from F. lugubris to F. pratensis and all the individuals of F. pratensis sampled from the Pyrenees had haplotypes clustering in the lugubris clade. No phylogeographical divisions were detected in F. lugubris. Comparison of species-wide phylogeography between the two sympatrically distributed species of ant demonstrates a difference in phylogeographical structure that implies different vicariant histories. However, over most of the species' distribution ranges, similar signs of demographic expansion predating the last glaciation and the lack of phylogeographical structure were found in both the eastern phylogroup of F. pratensis and F. lugubris. This finding is highly consistent with the results reported for all other boreal forest animal species studied to date in Eurasia. Contraction of the distribution range of each species to a single refugial area at different times during the late Pleistocene and a subsequent population expansion seem to be an explanation for the lack of phylogeographical structure across most of Eurasia in species that are ecologically associated with the boreal forest.