Through the analysis of hydrographic data collected during 16 cruises (1975-1985), we identify and describe the seasonal characteristics and the permanent features of the circulation in the Catalan Sea (north-western Mediterranean). Two fronts follow the continental shelf break on the peninsular and Balearic sides, separating the central dense waters from the surrounding lighter waters of Atlantic and continental origins. One is mainly produced by strong salinity gradients (Catalan front) and the other by temperature gradients (Balearic front). Two currents are associated with these fronts: the southwest Catalan current and the northeast Balearic current. Strong vertical mesoscale motions have been observed on several occasions in the weakly stratified central area. An interpretation of these mesoscale phenomena is proposed in terms of cyclonic eddies generated by some kind of instability mechanism in the two frontal systems.