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Abstract: The average age at sexual maturity (ASM) is an important parameter for evaluating the reproductive potential or status of a population. South American sea lions, Otaria flavescens in Patagonia (Argentina) were exploited and reduced to less than 10% of pre-exploitation numbers. At present, the population is recovering at a rate of 6%. In this paper, we studied growth and age at sexual maturity of South American sea lions in the south-western south Atlantic by examining 219 individuals (females and males) collected between 1989-2008. Individuals were aged by counting growth layer groups in tooth sections, standard body length was measured and male and female reproductive organs were examined macroscopically and histologically to establish individual sexual maturity. Maximum recorded length for males and females was 264cm and 200cm, respectively, and maximum ages 19 and 21 yrs. ASM defined as the age where 50% of females are mature, was estimated at 4.8±0.5 years old, corresponding to a mean SL of 147cm, about 81% of their asymptotic length. First observed ovulation occurred during the 4th year, first birth may occur between 4 and 5 years old. Males physiologically mature between 4-6 years, but the size of the testes shows that all males became sexually mature by the age of 9 years when they reach a mean SL of 212cm, about 86% of their asymptotic body length. The present information on ASM and growth of O. flavescens will improve the development of population dynamics models, to investigate the impact of recovering sea lions populations on its marine environment, as well as its trophic interactions with commercial fisheries. [ABSTRACT FROM AUTHOR]