Crasssotrea brasiliana (Lamark, 1819) is an epibiont filter feeder bivalve of fast growth and fast gonadal restructuring that feeds by filtration of microalgae, these characteristics are mainly related to the amount and quality of the phytoplankton filtered by the animal. In Paraná coast several sites of cultivation of the species can be found in Paranaguá Estuarine Complex (PEC) and Guaratuba Bay. Therefore, this work had as objectives to evaluate the size and the phytoplankton species ingested by adults of the oyster C. brasiliana and to describe its reproductive characteristics (index of meat revenue and stages of gonadal maturation). Oysters were acquired monthly in the Municipal Market of Paranaguá from cultivation in the PEC. The oysters were measured and length values (L), height (H) and gross weight (GW) obtained. Gonadal maturation stages were evaluated through the macroscopic examination of the gonads and revenue (R) through the wet weight of meat. The digestive gland was removed for the microscopic analysis of the stomach content (identification and measurement of the phytoplankton). The highest value of the R average (12,79%) was in July 2007, when a great portion of the population presented full gonads, however the  predominance of individuals with full  gonads (partially full and full) occurred in periods of warmer waters. The main microalgae genera found were: Diatoms - Gyrosigma Hassall, 1845; Cyclotella (Kützing) Brébisson, 1838; Coscinodiscus Ratray, 1890 and Pleurosigma W. Smith, 1852; Dinoflagellates - Noctiluca  Suriray, 1836 and  Ceratium Schrank, 1793 with size of phytoplankton ingested varying from 5 to 260 µm.