The stoplight parrotfish, Sparisoma viride, changes sex from female to male. In most cases, sex change is accompanied by dramatic change in coloration, from the female-like "initial phase" coloration to "terminal phase" coloration that is associated with males. However, some males do not change color at the same time they change sex, becoming female-mimic males (termed initial phase males). Using radioimmunoassays validated here for use with plasma from S. viride, we examined the hormonal profile of stoplight parrotfish undergoing sex change. As predicted, females were characterized by undetectable levels of 11-ketotestosterone (11KT), moderate levels of testosterone, and high levels of 17 beta-estradiol (E2). Fish that were found histologically to be undergoing sex change had elevated levels of 11KT and decreased levels of E2. Testosterone appeared to be unaffected. Males had the highest levels of 11KT and testosterone and low levels of E2. Because initial phase males eventually change color, we were able to take blood samples from males (sex confirmed histologically) before, during, or after color change. Thus, we also examined the hormonal correlates of color phase change. Initial phase males, like females, had undetectable levels of 11KT, moderate levels of testosterone, and significantly higher levels of E2 than either terminal phase males or males with transitional coloration. During color transition, 11KT levels rose sharply and levels of E2 declined. Males with terminal phase coloration had the highest levels of 11KT and testosterone and low levels of E2. Finally, administration of 11KT induced precocious sex change and color change in functional adult females. These findings suggest that 11KT plays a key role in instigating natural sex and color change, but that once sex change is complete, 11KT is not necessary for spermatogenesis or spawning behavior.