The house mouse is one of the most widely distributed and successful mammals in the world (2). It has dull greyish-brown fur and the tail, which is the same length as the body, is thicker and scalier than that of other species of mice (3). It is accompanied by a distinctive strong 'stale' odour and its presence can easily be detected by means of its droppings (2). Forms of this species living in association with man ('commensal' forms) tend to be larger and darker than 'wild' forms, and have longer tails (3). The voice is a familiar high-pitched 'squeak' (3).