During a Lagrangian study, in which a drifting buoy was followed over 5 days in a cold water upwelled filament off the NW coast of Spain, herbivorous grazing by size-fractionated mesozooplankton was measured. Total phytoplankton grazed by the copepods varied from 27.5 to 52.5mgC.m-2.d-1, of this 82% was by the 'small' copepod size fraction (>200 to <500µm) and 18% from the 'medium' size fraction (>500 to <1000µm). Over the study period, phytoplankton standing stock averaged 802.9mgChl a.m-2, and herbivorous grazing by copepods equated to a daily removal of 3.9-6.9% of the total phytoplankton chlorophyll a standing stock, and 17.1-36.1% of the >5µm standing stock. Total and >5µm primary production rates averaged 325.9 and 76.4mgC.m-2.d-1 respectively, while copepod grazing measurements accounted for 13.7% of the total daily primary production, and 59.8% of the >5µm primary production. Over this same period, measurements of microzooplankton grazing by copepods were also made, the relative contribution of both these sources to diet has been assessed. Carbon ingestion for the non-carnivorous copepod community was composed of ~90.7% phytoplankton and ~9.3% microzooplankton. © 2001 Elsevier Science Ltd. All rights reserved.