Abstract: The problem of verification of the fundamental rule of operation of Acoustic Doppler Current Profiler assuming the uniform current field within the four acoustic beams is studied. Analysis of scattering differences observed in four beams allows us to conclude about the homogeneity or inhomogeneity of the flow field. Geometry of the measurement performed by the ADCP is presented. The distance between beams, sampling volume and vertical resolution are studied in connection with the system parameters and depth of beam penetration. The so called anomaly index is invented in order to find the areas of the strongest variability. Examples of the inhomogeneity of the acoustic field within the measuring setup are presented and analysed. Differences observed in the strength of echosignal obtained by ADCP (150 kHz) and ELAC echosounder (30 kHz) are also analysed. Some reasonable explanations are examined, like the resonance of gas bubbles included inside the bodies and directional response of animals.