The ratio of ¹⁸O to ¹⁶O in the shell material of bivalves depends on the ambient water temperature at the time the shell material was deposited. By analyzing samples of shell material taken sequentially from the umbo to the shell margin, we obtained the oxygen isotopic records from the shells of 14 sea scallops (Placopecten magellanicus) and compared the isotope data with the visible rings on the upper valve. Using generalized additive models, we show that ring formation is related significantly to water temperature, and that rings were typically laid down near the annual temperature maximum. Shell ring formation was generally annual, although 2 of the mid-Atlantic scallops appeared to have laid down 2 rings in 1 y. Some of the scallops appear to form new shell material and increase in shell height over the entire year for the first few years of life, and in later years reduce or halt accretion at the shell margin during the coldest temperatures. The isotopic records obtained from near the umbo of the shells suggest that all but one of the scallops were spawned in the fall.