During the years 1966/67 a comparative study of the chemical composition of oysters was performed on protein, water, ash, trimethylamine oxide, trimethylamine, crude glycogen, iron (Fe+ + +) , calcium, magnesium, total and inorganic phosphorus, with oysters coming from the lagoon regions of the State of São Paulo, namely Cananéia and Bertioga Channel (Santos), and from the State of Paraná. The oyster discussed here is the species found on mangrove tree stilt roots. It was determined as Ostrea arborea Chemnitz, by Dr. Hugo de Souza Lopes, Museu Nacional do Rio de Janeiro. Other zoologists have placed the present species in the genus Crassostrea. Appraisal of the analytical results of the oysters was made taking into consideration the reproductive cycle and the meteorological conditions of the three regions under study. A seasonal variation was observed regarding fat, crude glycogen, dry matter and energy content when related to fresh and dry matter, and of protein when expressed in relation to dry matter. The seasonal variation are related to the reproductive cycle of oysters and is probably influenced by water temperature variations which depend on the solar radiation incidence, and also by phytoplankton abundance. We endeavoured to determine the season when distinct phases of the oyster reproductive cycle occurs, fattening, ripening, gonadal and sexual products discharge, for each one of the three regions studied, as well as the most favorable time of the year for comsumption (winter and spring). This paper shows that oysters are a complementary source of food and income, that their production must be managed for an optimum return to the population inhabiting the lagoon regions of Brazil southern coast.