Abstract Because of the differential amplitude of climatic oscillations, species living at northern latitudes are subject to more frequent and more severe range oscillations than species at southern latitudes. As a consequence, northern populations should, on average, be phylogenetically younger and possess less phylogeographical structure than closely related taxa further south. To test these predictions, we studied the mitochondrial-genetic population structure of NW Palearctic Lesser Black-backed Gulls (Larus fuscus group [=LBBG], five taxa) breeding at temperate to boreal latitudes from Iceland to the Taimyr Peninsula. Results were compared with those previously obtained (Liebers et al. 2001. Mol. Ecol.10: 2447) for more southerly breeding Yellow-legged Gulls (Larus cachinnans group, six taxa from the Atlantic Islands to Mongolia). Sequences of the hypervariable region I (HVR-I) of the mitochondrial control region revealed low within- and between-taxon sequence divergence, little genetic variation, a shallow haplotype phylogeny and poor phylogeographical structure in LBBGs compared with Yellow-legged Gulls. Haplotype frequencies among the five northern taxa formed a stepped cline with significant gene flow restriction between the forms heuglini and fuscus, probably indicating a secondary contact with (partial?) reproductive isolation. Western forms of LBBG, among which mitochondrial gene flow appears unrestricted, show genetic signs of postglacial range expansion and population growth. The Larus fuscus group is derived from a cachinnans-like ancestral population, probably in the Aralo-Caspian basin, and spread from east (NW Siberia) to west within the Palearctic.