The aim of this paper is to study the polychaete composition from a wide area in the Argentinean shelf, between 36°S and 55°S, by means of samples collected from soft bottoms with Picard dredge, during the "Shinkai Maru" cruises developed in 1978-79. As the study area comprises both the Magellanic and Argentinean Biogeographic Provinces, we analyze if these results show differences in polychaete composition between both Provinces. A total of 816 individuals was studied; 29 families, represented by 70 taxa, were identified. The most frequent (36-57% sampling stations) and abundant families were Onuphidae, Nephtyidae and Ampharetidae, mainly represented by Kinbergonuphis dorsalis, Aglaopha-mus sp. and Ampharete kerguelensis, respectively. Cluster Analysis, SIMPER test and ANOSIM showed two different species assemblages in two well defined areas according to a nearly 60m depth boundary, in correspondence with the classical Magellanic and Argentinean Provinces.