Egg quality is one of the most important factors determining larval viability. The study of oocytes released by 'wild' broodstock will contribute to the definition of quality criteria applied to hatcheries as well as being an indicator of the oyster reproduction potential and aquaculture ongrowing systems. The aim of this study was to determine the relationship between condition of Crassostrea gigas spp. angulata (Portuguese strain) from 'wild' broodstock (condition index, gonadal maturity and number of oocyte released), oocyte biochemistry and hatching rate of eggs during the spring/summer of 1995. Stages of gonadal maturity and condition index indicated that natural spawning of this species occurred between April and August with the highest spawning intensity occurring in June. Analyses demonstrated that C. gigas oocyte organic matter consist of 44-74% proteins, 16-38% lipids and 7-12% carbohydrates. A significant relationship was observed between some biochemical parameters - organic matter and lipid content - and broodstock condition index and hatching rate. It seems that these two oocyte parameters (organic matter and total lipids) could be used to define quality of oocyte and thus larval viability. The oocyte protein content was also found to be associated with broodstock and larval quality. In summary, these data are a contribution to the knowledge of the reproductive cycle of oysters in Europe and may be useful in improving hatchery management of this species. (C) 1999 Ifremer/Cnrs/Inra/Ird/Cemagref/Editions scientifiques et medicales Elsevier SAS.