Size-fractionated chlorophyll-a concentrations and carbon fixation rates in the euphotic zone of the shelf waters of NW Spain were examined using data collected between 1984 and 1992. The distribution of phytoplankton biomass and primary production values showed significant differences between the northern (Rias Altas) and the southern (Rias Baixas) regions of the shelf, with higher values in the Rias Baixas area during high production periods. Marked seasonal changes in both productivity and biomass resulted mainly because of the dominance of net phytoplankton (cells and colonies larger than 12 mm) relative to nanoplankton (cells and colonies between 0.8 and 12 mm) during high production periods. Samples were sorted in a sequence of decreasing productivity and biomass, with the highest values during bloom and summer upwelling periods, and lower values in stratified and winter-mixing periods. The range and the mean biomass and production rates were similar for blooms originated during spring or autumn and those induced by coastal upwelling during summer. Netplankton contributed a lower fraction of phytoplankton biomass during the initial phases of upwelling than in aged upwelled waters. However, its contribution to primary production rates was similar in both periods. Netplankton production rates were related mostly to the existing biomass, and the availability of radiation for photosynthesis was of secondary importance. Netplankton biomass and production were positively correlated with diatom abundance. However, there were no clear relationships between seasonal changes in biomass or production and the relative abundance of flagellates.