Postmortem biochemical changes and their relationship to the eating quality of surf clam muscle were studied. Shear value and cook loss continuously increased with the length of storage time up to 9 days postmortem and the rate of increase was much greater after 4 days of storage. A rapid breakdown of high energy phosphate compounds and muscle glycogen with a concomitant fall in pH after 4 days postmortem suggested that rigor onset occurred at 4 days and was completed between 6 and 9 days postmortem. The biochemical changes in surf clam muscles were much slower than those reported for the muscle systems from other species and were in a close coincidence with the change of shear values and cook loss. Another significant finding was that pyruvate instead of lactate was accumulated as glycolysis proceeded.