The mouse has black eyes, small peaked ears, and is fairly uniform in colour: mainly brown, with a lighter shade of fur on its underside. It is generally twice as heavy as field mice found on the mainland, with a mass of between and , and has longer hair and a longer tail. The evolution of a larger size has been credited to a lack of predators in its island habitat, which allows the mice to grow larger to preserve heat and increase fat storage. The mouse can reach a maximum length of about . It is found across the main island, Hirta, especially in the remains of human settlements, as well as on the island of Dùn. It is not found on Boreray. Studies of the fur of the mice have recorded the flea species Ctenophthalmus nobilis, and Nosopsyllus fasciatus on the mouse as well as the mite Typhloceras poppei. Studies of the intestines have observed the nematode Tictularia cristata and the cestode Hymenolepsis diminuta.