We investigated the hemolymph acid–base status of the hard-shelled mussel Mytilus coruscus exposed to air for 24–48 h at 24℃. M. coruscus exposed to air showed a decrease in hemolymph pH from 7.625 to 7.118 after 24 h and to 6.702 after 48 h. The hemolymph total CO_2 concentration increased from 1.43 mM/L to 3.12 mM/L during the first 24 h of air exposure, and increased to 6.32 mM/L after 48 h. The hemolymph CO_2 partial pressure increased from 1.50 torr to 9.48 torr during the first 24 h of air exposure, and increased to 41.8 torr for 48 h. The hemolymph [HCO_3^–] increased to 2.74 mM/L at 24 h and to 4.62 mM/L at 48 h. These results indicated that during prolonged air exposure M. coruscus showed hemolymph acidosis with partial compensation. M. coruscus were immersed in seawater after air exposure for 24 h, and the acid–base status recovered to initial levels within 3–24 h.