During October 1977, phytoplankton iamples were collected along with other hydrographic and biological data from a grid of stations located off Galicia (NW of Spain), between the coast and about 100 km offshore. Chlorophyll a distribution and phytoplankton composition were studied and related to hydrographic and geographic features of the area. The higher phytoplankton biomasses were found off the western coast and were conditioned by the presence of the rias - flooded tectonic valleys of moderate depth functioning as positive estuaries - and the occurrence of coastal upwelling, which allows the injection of nutrient rich water along the bottom of the rias. The qualitative composition of the phytoplankton reflected principally the influence of the rias but appeared also to show variability related to the distribution of water masses. The patterns of circulation of the surface and sub-surface waters were likely to reinforce the phytoplankton differences between the northern and western coasts of Galicia.