Particulate and dissolved organic carbon data, collected during five years in the western basin of the Mediterranean Sea, are presented and discussed. The influence of the river inputs is identified by higher surface DOC concentrations. Particulate carbon is mainly correlated with primary production in surface waters, and exhibits a seasonal variability. In surface waters, DOC is in higher concentration compared to deep water, in direct relation with particulate carbon, i.e. with production. Accumulation of DOC occurs during summer and disappears rapidly after September. Sections drawn along transects show that different water masses are identified, with characteristic DOC concentrations. Mechanisms of carbon cycling are identified, as are the general characteristics of the northwest Mediterranean Sea concerning organic carbon distribution. (C) 1997 Elsevier Science Ltd.