Juveniles of the Pacific oyster Crassostrea gigas were reared on a monospecific microalgal diet in order to quantify their 13 C and 15 N diet-tissue fractionations. The weights of these juveniles increased by up to 18-fold within 33 d. The juveniles reached isotopic equilibria with the diet, enabling calculations of fractionation values. The 13 C fractionation for tissues containing lipids ranked as 0.6 ‰ (gill lamella) > 0.3 ‰ (adductor muscle) >-0.2 ‰ (mantle lobe) >-0.9 ‰ (whole soft body) >-2.2 ‰ (midgut gland), while the 15 N fractionation ranked as 8.7 ‰ (adductor muscle) > 6.5 ‰ (mantle lobe) > 5.4 ‰ (whole soft body) > 5.2 ‰ (gill lamella) > 2.3 ‰ (midgut gland). Removal of lipids shifted the diet-equilibrated δ 13 C and δ 15 N values in all tissues except the adductor muscle, with resultant increases in the 13 C and 15 N fractionation values. The expected annual mean δ 13 C value for the diet of the oyster in the field is-17.0 ‰, which is an intermediate value among the δ 13 C of coastal phytoplankton (-20.2 ‰), epilithon (-20.0 ‰), epipelon (-14.8 ‰), and seaweeds (-14.9 ‰), suggesting that the oyster feeds on a mixture of these micro-and macroalgae. The expected δ 15 N diet value is 3.1 ‰, which is more depleted than values for micro-and macroalgae, suggesting that the 15 N fractionation in the field is smaller than that obtained from the feeding experiment.