Acoustic Doppler current profiler (ADCP) data collected during August-September 1991 reveal the diel migration of Zooplankton in the northeast Atlantic (50-60°N, 10-40°W). Volume scattering strength has been calculated, from which the speed and depth of migrations have been studied. There are usually at least two layers displaying nocturnal migration, one spending the day at depths of 300-400 m and the other at depths of 50-100 m. Reverse migrations are also found to be a common occurrence in the region studied. Usually, a surface layer begins to descend at dusk as soon as the upward migrating layer arrives in surface waters. Vertical velocity measured from the ADCP provides the first detailed direct measurements of the swimming speeds of the populations in situ, which are generally between 2 and 6 cm s-1. Migrating animals within layers do not move in unison; the animals at the leading edge are moving back towards the centre of the layer.