Abstract: Horizontal fish migrations in the Sulejów reservoir were studied acoustically. The SIMRAD EY500 echo sounder was used with transducer looking horizontally at three sites of the reservoir located along its longitudinal axis. Fish began to migrate horizontally in June and stopped in October. The diurnal cycle was very well pronounced, with fish going to open water at night and coming back to littoral in early morning. The timing of migrations was correlated with light conditions; the longer were nights the longer fish stayed off shore. Starting from August, apart from diurnal migrations between pelagial and littoral also some movement of fish towards the upper part of the reservoir was noted. It has been hypothesised that it was caused by unfavourable environmental conditions in the lower part of the reservoir, where the Microicystis aeruginosa bloom was observed, and the oxygen concentrations were much lower than in the upper part. These observations confirm that acoustics provides a good tool for controlling behaviour of fish, that are essential indicators of the water quality.