Abstract. 1. Distribution changes brought about by climate change are likely to alter levels of hybridisation between related taxa, and may threaten some species.
2. Nuclear (Tpi) and mitochondrial (cytB) DNA sequence data give evidence for introgression between two related Polyommatus (subgenus Aricia) butterfly species in a 150–200 km wide overlap zone in northern England and North Wales. A history of hybridisation is evident from the mixture of genotypes present within this region: some populations contain southern-origin (Polyommatus agestis) mtDNA and northern-origin (Polyommatus artaxerxes) Tpi alleles, and many populations contain mixtures of Tpi alleles.
3. The timing of the original hybridisation is unknown, but could be immediately post-glacial or much more recent in origin.
4. Both species are now beginning to shift northwards, associated with recent climatic warming.
5. It is thus expected that anthropogenic climate change will unleash a new spate of hybridisation, potentially threatening the long-term survival of the northern species in Britain.