Abstract: Social control of sex determination has been reported in juvenile Midas cichlids, Amphilophus citrinellus and was thought to be a heterochronic variant of functional sex change at the adult stage, as observed in some marine fishes. Large body size relative to group-mates was interpreted to cause male differentiation. In order to test the hypothesis that relative body size and behavioral interactions affect sex determination, thirteen experimental groups of juvenile Midas cichlids were raised to maturity. Fish were predicted to differentiate as males if they were larger than most of their group-mates and if they performed more aggressive behavior than they received. Several individuals were isolated and predicted to differentiate as females due to absence of conspecific stimulus hypothesized to be necessary to induce male differentiation. Neither relative body size nor behavioral interactions were found to affect sex determination. These results raise doubts concerning the reports that originally claimed that sex is socially determined in Midas cichlids. Greater body size typically observed in adult males appears to be due to faster post-maturational growth rather than relatively large juveniles differentiating as males.