Brooding pattern and fertility of the puelche oyster (Ostrea puelchana) were investigated in a native population from the San Matías Gulf (40°48′S; 65°05′W, Northern Patagonia, Argentina). Monthly samples of 100 oysters were randomly collected at the oyster ground during the period November 1999 to January 2000, and weekly in the 2000 to 2001 reproductive season. Complementary data were obtained from the literature (1976 to 1978) and samplings performed during the periods 1980 to1984 and 1987 and 1998. The date at which oysters begin larval brooding may be placed somewhere within the period November 18 to January 11. In most seasons (77%), brooding starts within the period November 18 to December 5. The number of brooders is low at the debut (7.5% in 1999, 4% in 2000), peaking at the end of spring (20%), and gradually decreasing until reaching nearly 1% by middle February. Total number of larvae brooded on each sampling date showed a high correlation with brooding percentages (r² = 0.85). Mean fertility showed no correlation with the percentage of oysters brooding larvae (r² = 0.12). The highest value of mean fertility (2.7 million) was recorded at the beginning of the season when 4% of the oysters were brooding larvae. Mean fertility ranged from 900,000 to 2,700,000 larvae. Brood size ranges from 447,500 to 3,790,000 larvae (mean = 1,868,212; s = 813,808; n = 121). Individual fertility showed no correlation with neither size (total height) nor with internal volume of oysters within a size range of 60–115 mm. O. puelchana shows the highest fertility recorded in Ostrea species, a feature that seems consistent with its short incubation period, small egg size, long planktonic life, and small size of pediveligers.