We investigated the use of δ¹³ C and δ¹⁸ O stable isotopes in otoliths of juvenile tainha (Mugil liza) as indicators of the stock structure of this species of mullet in the southern Atlantic Ocean. Our analysis identified 2 different spawning stocks along the Brazilian coast: the southern stock, from São Paulo (25°S) to Chui (33°S), with distinct seawater temperature requirements for spawning (18–21°C), and the northern stock, from Rio de Janeiro (23°S) to the north (up to 19°S), spawning in seawater temperatures of 21–24°C. These results will contribute to the development of appropriate stock management measures.