During three cruises, carried out in March 1991, October 1991, and January 1992 off the Iberian Peninsula and Morocco, the abundant calanoid copepod Calanus helgolandicus (Claus) was collected from a depth of 1000 m to the surface. Differences in depth preference were correlated with the life stage and geographically differing vertical salinity structures. In autumn and winter, only stage V copepodids (CV) and adults were found, in spring also younger copepodid stages. Within the range of the Mediterranean outflow water (MOW), a sharp decline of abundances of all stages was evident during all cruises. In autumn 1991, the bulk of the population was recorded south of the MOW, in winter 1992 north of it. During winter, numbers had declined by 70%, supporting the idea that winter individuals represent the same generation as was encountered in autumn, and that they had been transported northwards. CV stages preferred the depth layer 400-800 m, in autumn and winter. Adults were found in autumn at the same depth south of the MOW, while they preferred the 0-400 m layers north of it. In winter, the abundance of adults increased, males preferred the 400-600 m depth layer, while females stayed at 200-400 m. In spring 1991, stages younger than CV were found in high densities, all stages concentrating in the upper 200 m. During the crossslope survey in spring off Portugal, an absolute abundance maximum of females was found. In contrast, offshore densities were very low. On the basis of these findings, the hypothesis of a Mediterranean centre of distribution and dispersal into the Atlantic is questioned. It is suggested that a separate, reproductively active population of C. helgolandicus exists off NW. Africa.