Zooplankton were collected from five depth strata (0-25-50-100-200-500 m) at 12 stations along two sections across the Nansen Basin of the Arctic Ocean in July/August 1987. Vertical and horizontal distribution of biomass and abundance of the four dominant copepod species Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa, together with their gonad maturity state, are presented. Egg production of C. finmarchicus, C. glacialis and Metridia longa was measured from concurrent Bongo net samples in the upper 80 m. Four biological provinces were distinguished, which closely agreed with the hydrographic regimes. In the south, the Barents Shelf slope and the Southern Transition were located in the boundary current system of Atlantic water flowing eastward along the Eurasian shelves. They were characterized by the dominance of Calanus finmarchicus. The Barents Shelf slope, representing the core of the boundary current, hosted large populations of C. hyperboreus and Metridia longa. Metridia longa, with its main occurrence in the depth of the Atlantic layer, was the biological marker of this province. The southern provinces were separated from the Central Nansen Basin and the Nansen-Gakkel Ridge by a sharp discontinuity in biomass, stage distribution and gonad maturity of the three Calanus species north of 83°N. This faunistic boundary coincided with a well defined hydrographic frontal zone. Apparently, zooplankton distribution followed closely the flow of Atlantic water below 60-200 m eastward, although the three Calanus species had their center of abundance in the surface layer consisting of polar water, which supposedly follows the Transpolar Drift westward. © 1992 Pergamon Press Ltd.