Postlarvae of two atherinid species, Menidia menidia and Menidia beryllina, were tested in two 14-day experiments at four densities of fish to determine effects of interspecific versus intraspecific competition for food. In one experiment, when M. menidia was slightly larger than M. beryllina (simulating a slightly earlier hatching time in nature) M. menidia exhibited a clear competitive advantage over M. beryllina, as demonstrated by covariance analysis of fish biomass change on fish density. In the second experiment, when M. beryllina was larger than M. menidia, neither species showed competitive advantage. The results suggest that M. menidia has an inherently superior ability to compete for food. In another set of experiments, post-larval and juvenile M. menidia and M. beryllina were fed varying rations of Artemia nauplii during a series of three 10-day feeding trials. As Menidia menidia grew, they required a smaller percentage of body weight per day in food for maximum growth.