Most bivalve shells contain macroscopic growth-increment patterns which form with an annual periodicity. These are best observed in radial shell cross-sections and consist of alternating dark (translucent in transmitted light) and light (opaque in transmitted light) increments. A couplet of dark and light increments constitutes one year's shell growth and subdivides the continuous shell growth record into successive, annual components. Conventional wisdom suggests that dark increments represent slow shell growth associated with cold winter temperatures. Such a pattern characterizes shells of the much-studied hard clam, Mercenaria mercenaria, from coastal New England. Investigations of other species indicate, however, that not all dark increments form in response to cold temperatures. Yearly cycles in oxygen isotopic data from fossil and modern bivalves demonstrate considerable variability in the season of growth increment formation. Isotopic data suggest Jurassic Gryphaea arcuata from Great Britain formed dark increments during the warmest season, as do many other bivalves. In fact, oxygen isotope profiles across the shells of modern and fossil Mercenaria spp. from the US. east coast and the Atlantic and Gulf Coastal Plains indicate that in northern latitudes the dark increment typically forms during the winter season whereas in lower latitudes it forms during the summer to early fall. A transition zone occurs in the Middle Atlantic region where aspects of both patterns are observed. Water temperature extremes (high or low) appear to be the primary factor controlling the timing of dark (translucent) increment formation. Because of this dichotomy in the season of growth-increment formation, high-resolution isotopic sampling is invaluable when interpreting shell records in sclerochronological contexts, particularly for fossil bivalves.