Abstract The diet composition and feeding strategy of the New World silverside Odontesthes argentinensis were evaluated in a temperate coastal area during austral summer. This species is an economically significant resource for the fishing industry in South America. The food preference of O. argentinensis varied with its size-class: the smallest size-classes preferred diatoms and annelids, larger size-classes seemed to prefer barnacle cypris larvae, and the largest size-classes showed a preference for decapod megalopa larvae. Plankton samples were dominated by calanoid and cyclopoid copepods; however, contrary to what was expected, O. argentinensis showed negative selectivity for both groups. A selectivity index indicated that O. argentinensis selects the food it eats with regard to prey availability in the environment. The information obtained in this study demonstrates the importance of knowing the food preference of O. argentinensis to obtain a better condition factor, which will be key to its potential farming in closed systems.