Holding quahaugs and oysters through a growing season in circulation-controlled trays in Bideford river, Prince Edward Island, showed that they react differently to decreased water circulation. Quahaugs grow in proportion to the extent of circulation and can survive and grow significantly when there is very little water flow. Oysters are affected little by moderately reduced circulation but grow little and may die when the circulation is much reduced. Growth of quahaugs in beds is seriously reduced by eel-grass growing on the beds, and also by eel-grass merely in the vicinity of the beds. Growth curves are given for quahaugs in natural up-river and down-river beds and the difference in growth rate is attributed to difference in water circulation.