ABSTRACT: Marked changes in the composition and structure of the non-copepod components of a zooplankton assemblage were observed during the course of a 6 d drogue study in the southern Benguela ecosystem. These changes reflected in part the spiralednature of the drogue and were linked to the settlement of meroplankton from the surface waters and to the nocturnal recruitment, and subsequent accumulation, of migratory holoplankton and zoobenthos from depth. The presence of large numbers of benthicspecies in the plankton is hypothesised to reflect their upward migration at night owing to food limitation, since little of the overlying diatom bloom was sedimenting out. The presence of low-oxygen bottom water is hypothesised to account for the failureof the detritivorous zoobenthos to return to the seafloor during the day. The results suggest that the composition of zooplankton assemblages in upwelling areas is not only driven by the dynamism of the physical environment, but that, under certaincircumstances, a more subtle interplay between the biological and physical environment may result in strong changes in assemblage composition.