We examined hemolymph O_2 partial pressure (Po_2), pH, total CO_2 content (Tco_2), CO_2 partial pressure (Pco_2), and bicarbonate concentration ([HCO_3^–]) in order to evaluate the ability of the acid–base balance of the Pacific oyster Crassostrea gigas in normoxic conditions. Hemolymph was collected anaerobically through a cannula inserted into the adductor muscle of Pacific oyster submerged in experimental seawater. The mean values of hemolymph Po_2, pH, and Tco_2 were 62.0 torr, 7.414, and 1.87 mM/l, respectively. The apparent dissociation constant of carbonic acid (pKapp) was estimated using the following equation: pKapp = 33.462 – 13.032 • pH + 2.065 • pH^2 – 0.1088 • pH^3. Using αco_2 (40.51 μM/l/torr) and pKapp determined in this study, the hemolymph Pco_2 and [HCO_3^–] were calculated as 2.18 torr and 1.78 mM/l, respectively. The non-bicarbonate buffer value (β_NB) was 0.732 Slykes. These hemolymph properties were compared with those of other marine bivalves. The Pacific oyster should have an acid–base balance that is similar to Pectinidae and Pteriidae bivalves, but which is different from Mytilidae bivalves.