1. 8515 specimens of the common periwinkle, Littorina littorea (L.), from four localities on the North Yorkshire coast were examined for larval Digenea during 1966 and 1907. Four species were found, namely Cryptocotyle lingua (Creplin, 1825), Himasthla leptosoma (Creplin, 1829), Cercaria lebouri Stunkard, 1932 and Cercaria A, the incidence of infection with which correlated with the presence of sea bird and wader final hosts.
2. 5878 of all L. littorea examined were from one locality of relatively high incidence of infection where there were seasonal changes in incidence, being greatest during the autumn and early winter and least during the summer. The incidence of infection was higher in larger than in smaller periwinkles except in infections with an unidentified species, named Cercaria A, where the incidence declined sharply in L. littorea above 25 mm shell length. The ecology of the infections is discussed.