The life cycle and seasonal population changes of Janaira gracilis Moreira & Pires, is studied in detail. J. gracilis is the most common isopod of the phytal fauna of Sargasswn cymosum from Praia do Lamberto, Ubatuba, State of São Paulo (23º30'S and 45º07'W). The samples were collected once a month from January 1972 to January 1973. The data show a reproductive period continuous throughout the year with two peaks of major reproductive intensity. It is also discussed the populations characteristics and the sex ratio fluctuations.