1. Measurements were made under controlled conditions to determine the contribution of the swimbladder to the reflexion of acoustic energy from the perch, Perca fluviatilis.
2. The amplitude of the echo fell by 50% when the swimbladder was emptied; this was observed both when the dorsal surface and when the lateral surface faced the direction of the incident sound energy.
3. The polar diagram of the fish with its natural swimbladder is similar in shape to that obtained with a false ‘Onazote’ swimbladder.