Social control of sex determination has been reported in juvenile Midas Cichlids, Amphilophus citrinellus, held in large captive groups. In adults, males are typically larger than females. Large body size relative to group-mates was proposed to cause male differentiation, and small body size thought to cause female differentiation. More recent evidence has found no association between body size and sex in juveniles, a pattern inconsistent with the hypothesis that sex determination is controlled socially in this species. The current experiments were performed in order to determine if larger body size in adult males than in adult females is due to faster post-maturational growth, rather than due to larger juveniles differentiating as males and smaller juveniles differentiating as females. Juvenile Amphilophus cichlids from four lineages were divided into experimental groups, individually marked, and raised to maturity. Relative body size did not control sex determination. The observed growth patterns indicate that differences in body size between the sexes in adults is due to faster post-maturational growth in males than in females.