The Patagonian scallop Zygochlamys patagonica occurs on the SW Atlantic shelf of South America between 36�S to 55�S in beds at depths around 100 m. Stable oxygen and carbon isotope ratio analysis in scallops from four large beds (Uruguay, 36�17#S, Reclutas, 39�20#S, Tango B, 42�30#S, and Beagle, 55�10#S) in combination with condition indexes and oceanography date strongly suggest that shell growth increments in this species are formed annually. Most translucent growth bands coincide with low values of both d13C and d18O(i.e., they are formed at times of high remineralization activity and of higher water temperature). This pattern is consistent throughout the distributional range. Given the specific Argentinean shelf oceanography, higher temperatures at this depth occur during austral autumn—beginning winter in the region of Reclutas and Tango B but during summer–autumn at the northern and southern limit of the distributional range. Year-round investigations at Reclutas beds indicate that translucent shell growth bands coincide with a pause in somatic growth during austral winter, when energy investment is shifted from somatic growth to gonad development.