Growth-increment and isotopic studies of shells of the marine bivalve Glycymeris americana are a potential source of information bearing on its life history and preferred environment over the late Cenozoic on the U.S. eastern seaboard. We demonstrate that the ages of shells can be determined from growth bands and ontogenetic profiles of oxygen isotope (δ¹⁸O) composition, and that shell aragonite is deposited in oxygen isotopic equilibrium with seawater, enabling calculation of ambient temperatures by means of a generic transfer function. Modern specimens from North Carolina rarely reach the large size commonly attained by modern forms from Florida and Early Pleistocene forms from both states, and modern populations from North Carolina probably include fewer old individuals, the most certain disparity being with Early Pleistocene populations from the state. The temporal change in age structure in North Carolina may be an effect of recent scallop trawling but earlier non-anthropogenic environmental change cannot be ruled out as the cause. Maximum and minimum temperatures calculated from the δ¹⁸O profiles of Early Pleistocene shells indicate a larger seasonal range than now in both Florida and North Carolina, due to cooler winters. This may reflect greater southward penetration of cool northern waters, with transport along the shelf supplemented by upwelling of water brought south at depth.