Age structure and growth of shells of the Pacific basket-cockle, Clinocardium nuttallii in Netarts Bay, Oregon, were investigated with special emphasis on the differences between subtidal and intertidal populations. The cockle was chosen for study because its growth is highly sensitive to physical factors in the environment. Evidence is presented which suggests that stresses of the physical environment and biological factors result in oldest individuals being in the subtidal, and that growth of shells is correlated with the amount of tidal submergence. A short-term tagging study was done at two sites and most of yearly growth occurred in the Spring months. Collections were made from July, 1975, through September, 1976, from six sites to study synchronism of gonadal development and fecundity of populations by histological examination of gonads. Synchronism of gonadal development to the ripe stage occurs among intertidal and subtidal populations in the same area, while populations from different areas of the bay do not reach the ripe condition at the same time. There is evidence of protandric spawning behavior, and the hermaphroditic nature of C. nuttallii is compared to that of the Family Tridacnidae. The fecundity of the subtidal populations, in terms of percentages of the populations producing ripe gonads, was greater than fecundity of the intertidal populations. The subtidal environment is suggested to serve as 'refugia' for adult breeding populations. / Graduation date: 1978