Abstract: The aim of the study was lo determine susceptibility and avoidance behaviour of same freshwater fish exposed to underwater noise emitted by motor boats, most of all by the fishing boats with a Mariner motor (200 HP). The experimental station lo study fish responses was located in Lake Narie. Pikeperches, breams and carps were placed in a net cage and exposed to noise emitted by an underwater loudspeaker, noise duration and its characteristics being pre-established. Fish behaviour was registered on a video tape and stored in form of files related lo selected sequences. Pikeperch and bream did not show significant behavioural changes compared to the control periods. Carp, on the other hand, showed a noticeable and spontantaneous reaction, consisting of a 6-fold increase of the swimming speed. Adaptation period to noise of maximal volume was about 20 sec. It can be expected that in the conditions of the Vistula Lagoon, the minimal fish reaction will take place at a distance of about 150 m, while escape reaction should occur at 20 to 50 m. It is proposed to limit the speed of motor boats over fish spawning grounds and migration routes.