A total of 722 Coastal Zone Color Scanner (CZCS) images obtained during 1979-1985 were used to investigatethe variability of surface phytoplankton in the Portugal Current System off western Iberia. Seasonal fluctuations of pigment biomass are clearly identified with amplitudes and phases varying from the shelf offshore and also with latitude. Interannual fluctuations are also present. The spatial distribution of phytoplankton varies strongly with topographic constraints and dynamical factors, such as the topography of the shelf and upper slope, pronounced coastline features, stratification due to the heating-cooling seasonal cycle and to river runoff, fronts, coastal upwelling, mesoscale filaments and jet-like structures, and surface currents. Surface pigment biomass is generally higher in the north, mainly in relation to the Galicia Front, and on the shelf, especially in winter (due to stratification and nutrient enrichment induced by river outflow) and in summer (as a result of coastal upwelling). Wavenumber analysis reveals a strong influence of mesoscaleand submesoscale processes on phytoplankton growth and aggregation. The relevance of this study for the global carbon cycle and for fisheries is briefly described.