Abstract
1. Maritrema apodemicum sp. nov. from the long tailed field mouse, Apodemus sylvaticus sylvalicus (L.) on Skomer Island is described and compared with other species of the genus.2. 14·6% of the mice were infested, the % infestation being greater in the males, 20·3%, than the females, 5·4%.3. A brief discussion of the genus and of other species occurring in mammals is included.