Polychaeta were used as indicator organisms for the study of some physical and biological conditions of the beaches of the northern coast of São Paulo, Samples were taken in 191 stations at different levels of the tidal zone, on 11 sandy beaches situated along Flamengo Bay (continent) and Palmas Bay (Anchieta Island). Sediment was analyzed for grain size distribution, porosity, organic matter and calcareous content. Qualitative and quantitative analysis of the Polychaeta fauna and measurements of temperature, dissolved oxygen and salinity of the interstitial water were made. Macrofauna was found to be completely absent from the beaches of clean coarse sand which may be due to the quite unstable condition of the sand, intensively revolved by waves action. The fine sand of many beaches offers more consistent protection for a number of burrowing animals. The Enseada and Grande beaches (respectively on the continent and on Anchieta Island) show the same very fine and homogeneous sand, and as we could expect are inhabited by the same species of Polychaeta. Nerine agilis, Arenicola brasiliensis and also Diopatra cf. cuprea are frequently found on those fine sand beaches. The most selective factors were salinity and organic matter content. The euryhaline species Laeonereis culveri and Nerine agilis, have a wide distribution along the intertidal belt. Their highest density was recorded in areas of salinity below 10‰. The muddy sand of Saco da Ribeira beach showed the highest number of Polychaeta species, the abundance of Laeonereis culvert and Diopatra cf. cuprea contributing significantly to the biomass of this area. The occurrence of high levels of hydrogen sulphide is a characteristic on the Saco da Ribeira beach, where the sand a few centimeters below the surface is almost quite black. The corresponding very low concentrations of dissolved oxygen seems to be insufficient for many species living inside the sediment. We assume that the Polychaeta species found in those spots probably migrate to surface layers where the diffusion could supply oxygen. The Saco da Ribeira and Enseada beaches showed some degree of pollution, which may have affected che selective distribution of the Polychaeta fauna.