A detailed study of 21Opo, the predominant alpha-emitting nuclide found in most marine organisms, has been undertaken in a particular zooplanktonic species, the euphausiid Meganyctiphanes norve~ca. The purpose was to obtain information concerning the origin, the localization and the flux of the nuclide in and through this organism. Measurements of 21Opo were made in euphausiids of different sizes, in dissected organs and tissues, and in excretion products. The results show higher concentrations in the smaller specimens; this fact cannot be explained on the basis of surface adsorption, but is probably related to the ingestion of food. Dissection results show that the distribution of 21Opo in euphausiids is not homogeneous, but that the majority is concentrated in the internal organs, the alimentary tract and the hepatopancreas in particular. The natural radiation dose received by these organs is in consequence much higher than that received by the whole animal. Use of a dynamic model allowed the flux of 21Opo through M. norvegica to be calculated. The calculations confirm that food is the principal source of 21Opo for this species, and clearly show that fecal pellets constitute the major elimination route. Extrapolation of the data to zooplankton in general leads to the conclusion that zooplankton metabolic activity plays an important role in transporting 21Opo from the surface layers of the ocean to depth.