Forty-four shore-normal transects along the Northwestern Mediterranean coast between the mouth of the Rhône River (France) and Valencia City (Spain) were sampled during the REDIT-I (September 1998, [R1]) and REDIT-II (December 1999, [R2]) campaigns. Polychaete distribution patterns on shallow littoral fine sands (10 to 50 m water depths) were analyzed at a regional scale. A total of 359 species of polychaetes were identified which represents 38% of all polychaete species recorded in the western Mediterranean. Four main soft-bottom communities were identified from the samples in the area: Littoral Fine Sands, Littoral Sandy Mud, Terrigenous Coastal Mud and Detritic Sand. Predominantly sandy environments were characterized by Ditrupa arietina and Owenia fusiformis while Lumbrineris latreilli, Hilbigneris gracilis and Sternaspis scutata were numerically dominant in muddy environments. Biological diversity assessments at different temporal and spatial scales are required by the European Marine Strategy Framework Directive (2008/56/EC) in accordance with criteria and methodological standards of Good Environmental Status (GEnS). Selected indicators for descriptors are explored based on this mesoscale assessment.