The age and growth of northern quahogs Mercenaria mercenaria (=hard clam) from inner Tokyo Bay, central Japan, were investigated by observing the annuli in their hinge teeth. The ages in the shallow and deep areas are 2–9+ and 1–13+, respectively. Significant differences in growth between clams found in the shallow and deep areas were detected. The growth coefficient and shell length of clams of a particular age in the shallow area were greater than those in the deep areas. Northern quahogs in both shallow and deep areas were confirmed to spawn around summer. A larger gonadosomatic index and a higher proportion of spawning individuals were found in shallow areas. These results indicate that shallow habitats provide enhanced conditions for somatic development in northern quahogs in this region.