The number of Polyplax serrata (Burm.) on the mouse Mus musculus L. is determined by the efficiency with which the mouse grooms itself with its mouth. The efficiency with which the accessible hindpart of the body is groomed is such that normally the majority of all stages of the life cycle are found on the forepart of the mouse with the exception of the stage I nymph which is distributed over the whole body. The principal requirements for self-grooming to control the number of lice are that the technique is efficient, that sufficient time is spent grooming, that an adequate area of the body is groomed, and that lice move readily into the accessible area. Any factor which influences adversely any one of these requirements causes the efficiency of grooming to decrease, and thus permits lice to increase in numbers and to populate the whole body.