The silverside (Odontesthes bonariensis), an introduced species, was studied in Laguna Grande of San Pedro (Laguna Grande) and Laguna Chica of San Pedro (Laguna Chica), both eutrophic lakes of the Biobío River basin in central Chile. The parameters analyzed were: biomass, production, length frequency, weight-length relationship, condition factor, age, and growthO. bonariensis, the most abundant fish species in the lakes, had high values of biomass and production, showing its adaptation to lakes of medium (Laguna Chica) and high (Laguna Grande) eutrophic status. Laguna Chica had greater biomass and production of silverside than Laguna Grande, confirming the hypothesis that the mesotrophic condition is more favorable than the highly eutrophic condition for certain species. In addition, the population of Laguna Chica reached an asymptotic length and a longevity that were greater than those of Laguna Grande. However, individuals of Laguna Grande were stoutter (better condition index), reflecting a higher availability of food in the eutrophic lake.