A survey of Skomer voles (Clethrionomys glareolus skomerensis) conducted in August of 1994 and 1995 discovered that 51% of live-trapped animals were positive for Cryptosporidium species (Coccidia). Of the positive animals, 85% were shedding C. muris in their faeces, 5% C. parvum and 10% apparently both parasite species. On the U.K. mainland, the prevalence of Cryptosporidium in the bank vole (Clethrionomys glareolus) is a quarter of that on Skomer and the species normally found is C. parvum. Interest in C. parvum stems from the often severe diarrhoeal disease crypt osporidiosis which it can cause in humans and livestock. The parasite occupies the gastro-intestinal tract and is transmitted between hosts by the faecal-oral route. It has been suggested that wild rodents may be an important reservoir of infection by C. parvum for livestock. However, on Skomer island, C. muris, which is rarely found in livestock, is the dominant species infecting voles.