The seasonal succession of phytoplankton assemblages in coastal, shelf and oceanic stations of the southern Bay of Biscay was monitored during 1987, and the information summarized by means of multivariate techniques (correspondence and concentration analysis). Microflagellates were the most abundant group throughout the year. In winter, oceanic and tychoplanktonic diatoms as well as chrysophyceans dominated the phytoplankton community. High densities of microflagellates in March, followed by a chain-forming diatom bloom in April were both associated with convergences driven by slope currents. Three distinct assemblages, almost exclusively dinoflagellates, succeeded during summer. The results suggest that the replacement of dinoflagellate groups throughout the summer could be related to variations in cell size as a response to the deepening of the nutrient-depleted level. Large diatoms known to host endosymbiotic cyanobacteria in other areas (Rhizosolenia alata alata and Hemiaulus hauckii) also appeared during the stratification period mainly in offshore waters. An assemblage of subtropical diatom and dinoflagellate species was found in late autumn associated with the intrusion of high salinity waters along the northern Spanish coast. The results of both multivariate approaches were consistent and demonstrated the existence of groups of species not strictly predicted by general models of phytoplankton succession in temperate seas. These assemblages, however, were closely related to the vertical and horizontal variability in the physical field. In this context, concentration analysis has proved to be a useful tool for finding out the underlying relationships between phytoplankton assemblages and the environment.