Abstract Adductor activity of the freshwater mussel Andonta cygnea was examined continuously for a period of 32 days under circumneutral and acidic conditions. The adductor activity and valve movements were monitored using a Mussel Observation Program (MOP) based on electromagnetic induction. Acute acid exposure disturbed the rhythmic valve activity by changing the normal pattern of successive active/inactive (open/closed) periods. Fluctuations in water pH were reflected in changes in mussel adductor activity. Obviously these changes in adductor activity induced by low pH have adaptive significance for mussels in water subject to episodic acidification.