Present age determination techniques for the sea scallop, Placopecten magellanicus (Gmelin), rely on the subjective interpretation of lines on the shell exterior as representing periods of annual growth. This study compares scallop age and growth estimates from the external line method with a stable isotope technique. The oxygen isotopic records from serially sampled carbonate powders taken from two scallop specimens collected alive off the Virginia coast show annual cycles which closely approximate the isotopic composition predicted as a function of observed salinity and temperature. Since these annual isotopic cycles are controlled by physical-chemical processes, they provide an independent time scale for age and growth rate determination. Growth rates determined from the isotopic records are roughly twice those estimated from the external line method and from a published average growth curve for Placopecten magellanicus.