The reproductive biology of Patagonian flounder (Paralichthys patagonicus) was studied in the Bonaerense Coastal Zone (34°-42°S) using samples collected in December 2003 and 2005. According to the literature, the data were analyzed considering two population groups: one in the northern zone (34°-38°S) and another in El Rincón (38°-42°S). The estimated size at first maturity for both sexes in 2003 was 31.57 cm TL for the northern zone and 29.58 cm TL for El Rincón. In 2005, the L50 was only estimated for the northern zone, and the value obtained was less than that of 2003 (27.82 cm TL). In December 2003, the greatest reproductive activity took place in El Rincón. However, in December 2005, the highest proportions of hydrated fe-males were observed in the northern area. This could be due to the cooler temperatures recorded for this zone, as compared with 2003. Batch fecundity showed a potential fit to size and a lineal fit to weight and ranged from 14,685 (30 cm TL) to 153,717 ( 59 cm TL) hydrated oocytes. Relative fecundity oscillated between 24 and 170 hydrated oocytes/g. The values of these parameters differed between years and among population groups. These differences could be attributed in part to the difference in seawater temperature between the two years, especially the high temperatures observed in 2005.