The incidence and degree of infestation of the Bank vole, Clethrionomys glareolus, the Skomer vole, C. skomerensis, the Common shrew, Sorex araneus and the Pygmy shrew, S. minutus have been studied from a rough grassland and woodland area, Aberystwyth and from Skomer Island in Wales. In addition, small numbers of the Short-tailed vole, Microtus agrestis and the Water shrew, Neomys fodiens were examined for helminth parasites, when they became available. Seven new host records and six new British records are listed. The number of species of helminths in shrews, which consisted largely of digeneans and cestodes, was greater than that in voles. This is undoubtedly linked with differences in the feeding habits of the two hosts.
In Aberystwyth, where the composition of the helminth fauna was found to be more varied than that from Skomer Island, one species of nematode showed evidence of seasonal variation in the degree of infestation of Clethrionomys glareolus and four species, one digenean, two cestodes and one acantocephalan in Sorex araneus. The factors affecting this seasonal fluctuation in parasite numbers are discussed.