Crassostrea gigas was introduced in Anegada Bay (North Patagonia, Argentina), in 1981 for aquaculture purposes. The species has, since established in the field, covered all available hard substrata in the bay and gradually expanded south along the coast of the neighboring province of Río Negro, 90 km away from its original introduction site. Our work focused on the reproductive dynamics of the species at the introduction site, with emphasis on the thermal thresholds needed for each stage of gonad development. During early spring, the oysters in Anegada Bay go through active gonad proliferation. Maturity extends mainly from October to January. The first spawning occurs in November, when seawater temperature exceeds 17°C, and peaks from December to February, when seawater temperature lies in the range ot 19–21.5^°C. The timing of gonad development is size dependent. Larger (≥70 mm) and medium-size oysters begin spawning first, whereas small oysters (<40 mm) represent the late spawners of the season. The C. gigas population in Anegada Bay is a clear and well-documented case of an introduction site where optimal environmental conditions are met, both for complete gonad maturation and for successful larval survival and settlement. We compiled a series of C. gigas cases from different parts of the world in which reproduction is related to seawater temperature. We concluded that the thermal threshold for full female gonad ripening in this species is strict (temperature > 17^°C), and spawning only occurs in those sites where this threshold is reached.