Size fractionation with 2000 and 1000 μm screens is used by the Institute of Marine Research in Norway in routine monitoring of zooplankton biomass. This study examines the separation of taxa by this procedure. For copepods and cladocerans, the fractionation separates individuals according to their size in a consistent and predictable manner. Individuals up to 0.4 mm in width are contained in the small fraction (<1 mm). From width 0.4 to 0.8 mm, there is a progressive shift from the small to the medium fraction (1-2 mm). From about 0.8 mm width, individuals start to be contained in the large fraction (>2 mm). For