A method for measuring the glycogen content of the hepatopancreas of Cepaea hortensis is described. Glycogen content was determined for snails from field population cages and from a nearby natural population. Results showed that caged snails possessed lower levels of glycogen than did the natural population, while the lowest levels of glycogen were associated with higher snail densities in the cages. There is evidence for an increase in glycogen content of C. hortensis during the late summer in nature and in experimental populations. The results are discussed with regard to the selective effect that glycogen may have on Cepaea and to the interpretation of events occurring in populations of Cepaea that glycogen assay could provide.