Parasitological data were used to test the hypothesis that the wood mouse (Apodemus sylvaticus) population of Fair Isle, Shetland, originated from the British Isles rather than Scandinavia, as is usually argued. This study was based on the assumption that the mice were likely to share most of their parasite fauna with conspecifics from their ancestral home. The ecto- and endo-parasites of wood mice on Fair Isle between 18 June to 10 July 1987 and 17 to 26 August 1991 were identified and compared with those reported from conspecifics in the two putative source areas. All eight species of metazoan parasites that infected A. sylvaticus on Fair Isle were common to mice of the British Isles, whereas just one parasite on Fair Isle, a mite, had been recorded from Scandinavia. This lends support to the hypothesis that the mice originated from Britain rather than Scandinavia.