Adult male Eastern Fence Lizards (Sceloporus undulatus) possess blue and black ventral patches that function in sex recognition and intrasexual social signaling, but this ventral coloration is absent or greatly reduced in females and juvenile males. Adult males also exhibit a relatively uniform, reddish brown dorsal coloration, while females and juvenile males are cryptically colored, with two rows of dark brown or black chevrons set against a background of gray and brown. In the present report, we show that sexual divergence in ventral coloration is temporally correlated with sexual divergence in plasma testosterone in free-living juvenile males and females, supporting the hypothesis that sexual dichromatism is regulated at least in part by testosterone. We experimentally tested this hypothesis by (1) removing the primary source of circulating testosterone in juvenile males via surgical castration, (2) restoring testosterone in castrated males with tonic-release implants, and (3) implanting intact juvenile females with exogenous testosterone. As predicted by our hypothesis, the development of blue and black ventral coloration in S. undulatus was (1) inhibited by castration in juvenile males, (2) restored by exogenous testosterone following castration in juvenile males, and (3) promoted by exogenous testosterone in juvenile females. The expression of male-specific dorsal coloration was also (1) inhibited by castration and (2) restored by exogenous testosterone following castration in juvenile males. Our results are consistent with established literature supporting the critical role of androgens in the mediation of sexually dimorphic coloration among phrynosomatid lizards.