Abstract: Acoustical monitoring of fish at an electric barrier was performed using EY 500 split beam echo-sounder with the transducer directed horizontally. Stationary measurements done during the operating barrier and when the electric power was switched off have shown statistically significant difference in the number of fish approaching the barrier, 11 fish in the first case, and over 280 in the second. The electric barrier affected more strongly large fish than the small ones. Mobile measurements performed in a basin from which fish were approaching barrier revealed the higher fish concentration just in front of the operating barrier than elsewhere.