[His⁷]-corazonin is a neuropeptide that was originally isolated from the American grasshopper, Schistocerca americana (Drury), without known function. The current study was carried out to examine the possible involvement of this peptide in the induction of dark color in nymphs of this grasshopper. Additionally, the effects of a homolog, [Arg⁷]-corazonin, and juvenile hormone (JH) III on body color were investigated. Light-colored nymphs obtained at a high temperature were used for bioassays of the peptides. [His⁷]-corazonin showed a dose-dependent induction of black patterns, but it did not influence the green-brown polyphenism. [Arg⁷]-corazonin that induced black patterns in the injected nymphs was as potent as [His⁷]-corazonin, indicating that the substitute of His by Arg does not influence the dark color-inducing effect. Injection of JH III into reddish nymphs obtained at a low temperature caused them to turn green in a dose-dependent manner, but it had no effect on the expression of black patterns. These results may suggest that [His⁷]-corazonin and JH are involved in the control of black pattern expression and green-brown polyphenism, respectively, in this grasshopper. This is the first study demonstrating a physiological function of [His⁷]-corazonin in this grasshopper.