Oxygen isotope ratios (δ18O) of three Pacific oyster (Crassostrea gigas) specimens from the Neolithic Yeondae-do shell midden site, Tongyeong, Korea, were analyzed to determine the seasonality of shellfish gathering and site occupation. Oxygen isotope samples were taken from the left valve hinge sections of the specimens. Oxygen isotope values ranged between - 0.1 ‰ and - 2.4 ‰, between - 0.2 ‰ and - 2.9 ‰, and between 0.3 ‰ and - 2.8 ‰ in oyster specimen #one, #two and #three, respectively. The isotope profiles showed seasonal temperature cycles, providing information related to the seasonality of shellfish gathering and site occupation. Hinge-edge oxygen isotope values of the specimens showed decreasing trends after passing through maximum values (winter), indicating that they formed during spring. Thus it can be assumed that during spring season, oysters were gathered and the site was occupied.