Horizontal and vertical velocities of fish schools were measured using an acoustic Doppler current profiler (ADCP). To determine three orthogonal velocity vectors (east, north, and vertical), it was required that the four ADCP beams simultaneously insonified a fish school, in the same depth bin. Velocity vectors which satisfied these conditions were extracted from individual ping velocity estimates and ensemble averaged to determine the average speeds and directions of fish aggregations. The results suggests that the ADCP can be a useful tool for observing fish behavior in certain situations. Some applications may include the quantification of horizontal and vertical migration patterns of large scattering layers and possibly vessel avoidance reaction. The method can be enhanced by utilizing the radial velocity components from each beam and correcting for platform motion.