Polychaete assemblages were studied from samples taken in the Southwestern Atlantic, between 34°S and 38°S (Argentinean Biogeographical Province) during 1993, 2001 and 2002 from a series of oceanographic cruises in mixohaline to oceanic waters. Polychaetes represented almost 25% (number of taxa) of the macrobenthic assemblages; 57 taxa corresponding to 29 families were identified. The MDS plot shows an ordination of samples according to the following areas: the central estuary and mouth of Río de la Plata (group 1), the coastal shelves of Uruguay and Argentina and the transect off the mouth of Río de la Plata (groups 2 and 3) and other coastal stations from both countries (groups 4 and 5). The abiotic variable that best explained the distribution pattern (BIO-ENV analysis) was the bottom salinity, while the best combination included 2 variables: bottom salinity and type of bottom. Trophic diversity increased with salinity. The distributional range of four species: Sabellaria bellis Hansen, 1882; Travisia amadoi Elías et al., 2003a: Terebellides lanai Solis-Weiss et al., 1991 and Aglaophamus uruguayi Hartman, 1953, was extended.