The St Kilda house mouse (Mus musculus muralis) was a subspecies of the house mouse found only on the islands of the St Kilda archipelago of northwest Scotland. It is uncertain when they first arrived on the islands, but it is possible that they unwittingly were transported there during the Norse period. Isolated on the islands, the St Kilda house mouse diverged from relatives, it became larger than the mainland varieties, although it had a number of traits in common with a subspecies found on Mykines in the Faroe Islands, Mus musculus mykinessiensis.