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Bank voles Clethrionomys glareolus Schreber, 1780 were first discovered in Ireland in 1964. They are confined to the south-west and, judging by their rate of spread, are a recent introduction. Mitochondrial DNA was extracted from 81 bank voles from 5 localities. Only 2 haplotypes were observed, indicating that the founder populations was small. There were marked differences in the relative frequencies of haplotypes between sites. These are most readily explained by local founder effects brought about by the habitat preference of this rodent and sustained by the territoroal behaviour of females.