Monthly collections were made for one year (August 1995 to July 1996) in the intertidal zone of two beaches (Engenho d'Água and São Francisco) located in the São Sebastião Channel, State of São Paulo, Brazil, in order to establish the seasonal fluctuations of the species and the factors determining these fluctuations. On each beach, a 10 m wide sector of varying width was established, from which 15 samples/month were collected with a cylindrical corer (0·01 m2). The salinity of the interstitial water and the temperature of the sediment were measured monthly, while the granulometric parameters and calcium carbonate and organic matter concentrations were recorded seasonally. The temporal distribution of the species was more homogeneous at Engenho d'Água, where the highest diversity and evenness values were also found, although the number of individuals was lower. At São Francisco, the monthly abundance was higher, but the richness, diversity, and evenness values were lower, showing some heterogeneity in the temporal distribution patterns of the species. Most species at Engenho d'Água reached their peak of abundance in summer and autumn. The most abundant and frequent species at Engenho d'Água were Nematonereis hebes, Cirriformia filigera (summer), Scyphoproctus djiboutiensis (spring), Owenia fusiformis and Isolda pulchella (winter). At São Francisco, the capitellids Capitella capitata, Heteromastus filiformis, and Capitomastus minimus, and the tubificid oligochaetes dominated in the summer, and Laeonereis acuta and Scolelepis squamata in the winter. The log-normal distribution of the species was used to group those species that are sensitive to and those that are tolerant of organic enrichment, over the four seasons of the year.