Four water masses have been recognized in the Central Cantabrian Sea (Bay of Biscay): Surface Water. Central Water, Mediterranean Water and Deep Water. Mediterranean Water flowed at about depth with a mean salinity value of 35.799 PSU and sigma-t between 27.58 and 27.59. The percentage of Mediterranean Water within this area was 39. The lower limit of Central Water was 500 m. The upper limit was variable throughout the year. In spring and autumn surface waters flowed in an eastern direction and Central Water characteristics were similar to those previously described for other Atlantic regions. During summer, superficial currents flowed to the West. A very homogeneous Central Water, Bay of Biscay Central Water (BBCW), was also found. Two features were related to the main circulation pattern: intrusions of high salinity Central Water in spring and autumn. whereas in summer Central Water upwelled at coastal stations. During summer, a stepwise thermocline developed between 20 and 50 m, as a consequence of intense surface heating. This thermocline did not usually separate two well differentiated mixed layers. Water of continental origin was distributed in a thin layer all over the study area during summer, whereas in spring and autumn it was found close to the coast. Two different stratification processes were recognized: a mainly thermal stratification in summer. and coastal stratification caused by salinity gradients the rest style='font-size:8.0pt'>of the year.