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.

1. 5250 specimens of the common periwinkle, Littorina littorea (L.), from Scalby Rocks on the North Yorkshire coast were examined from October 1966 to November 1967 and data obtained on the size, sex and reproductive condition of the periwinkles and their infections with four species of Digenea, namely Cryptocotyle lingua (Creplin, 1825), Renicola roscovita (Stunkard, 1932), Himasthla leptosoma (Creplin, 1829) and Cercaria lebouri Stunkard, 1932.2. The normal breeding cycle of uninfected L. littorea and the breeding cycles found in periwinkles infected with larval Digenea are described and discussed. Initial infection takes place mainly when periwinkles are spent after their first breeding season and though later damage to the gonad may prevent reproduction, the breeding potential of the periwinkle population at Scalby Rocks is unlikely to be affected seriously. There was no evidence of sex reversal in uninfected or infected L. littorea and the normal sex ratio is 1:1. The phenomenon of gigantism as a result of digenean infection is discussed with respect to L. littorea.

Summary1. Individual chemical estimations of the glycogen concentration of the digestive gland and the foot were made on 272 uninfected L. littorea and 395 L. littorea infected with larval Digenea from Scalby Rocks, North Yorkshire, during the period from September 1966 to November 1967.2. The glycogen concentrations of the digestive gland and the foot of both infected and uninfected L. littorea showed pronounced seasonal changes, being highest during the autumn and lowest in the spring. Infected L. littorea had less glycogen in the digestive gland and foot than had uninfected L. littorea, and the amount of reduction was characteristic for each of the three species studied, namely Cryptocotyle lingua, Renicola roscovita and H. leptosoma. Infection with C. lingua caused greater decrease in the glycogen concentration of the digestive gland, while infection with R. roscovita had a more marked effect on the foot. The effect of infection with H. leptosoma and with concurrent infections with two species are described and considered.3. The seasonal changes in the amount of glycogen in the digestive gland and the foot of uninfected and infected L. liitorea are considered in relation to seasonal reproductive changes and the ecology of the host at Scalby Rocks.