The aim of this study was to investigate the effect of CO₂-induced seawater acidification (pH 8.1; control pH 7.0 and 6.0) on the survival, behaviour and metabolic rate of the Baltic clam Macoma balthica during short- (36 hours) and long-term (14 days) exposures. Total metabolic rate and gaping behaviour (shell opening and closing) were determined based on heat dissipation measurements using an isothermal twin calorimeter of the Calvet type. The survival and burrowing behaviour during long-term exposure were observed every day. Short-term exposure to reduced pH did not significantly affect the resting or active metabolic rate. Neither were there any significant changes in gaping behaviour. However, long-term exposure significantly affected burrowing behaviour. In the lowest-pH treatment, both resting and active metabolic rates were also significantly higher than in the control.