In this work we describe a simple methodology to assess the availability of quality gametes throughout the year to perform technological assays. This methodology was applied to the evaluation of the reproductive seasonality of Crassostrea gigas from October 2007 to November 2008 in the western Mediterranean coast (Burriana, Spain). Gonad maturation degree and sex were assessed. When both gametes were present, fertilization assays were performed. The percentage of differentiated males increased from January to February (1%) to July to August (62%). Females presented the same tendency, also reaching maximum values in July to August (32%). Both males and females showed a significant degree of maximum gonad maturation in July to August. Both gamete types could only be obtained from March to October, normal larvae being observed only from July to October. Breeding periods established in this work coincided with those reported by other authors in different Mediterranean and Atlantic coastal regions. These results indicate that oysters from different origins and maintained in natural conditions are an alternative to performing laboratory studies or assays, but only in the months from July to October.