Abstract: Nowadays, each oceanographic vessel is equipped with an Acoustic Doppler Current Profiler. Its task is to determine the value and direction of the vector of water current. It measures the Doppler shift in the frequency of the acoustic wave backscattered by neutrally floating particles suspended in the sea water. A by-product of such measurement is proportional to backscattering strength SV. Starting with the pilot work by Flagg and Smith [3], an ADCP becomes a useful tool for the investigations of the distribution and migration of zooplankton. It enables current and zooplankton abundance to be obtained simultaneously. The presented paper is an attempt to exploit the data collected in a routine way by the ADCP mounted on r/v ‘Oceania’ to characterise the biological aggregations, and especially to estimate the distribution of zooplankton and to observe their diel vertical migration.