The effects of various environmental factors on body color were investigated for nymphs of the American grasshopper, Schistocerca americana (Drury). Two types of body color variation were separately analyzed. In so-called green-brown polyphenism, a green, yellow and reddish background body colors were recognized, though the variation was continuous. The reddish morph was common at low temperatures (e.g., 30 and 34°C), while the green and yellow ones became predominant at high temperatures (e.g., 38 and 42°C). The other factors including humidity, background color and rearing density had little influence on this polyphenism. The variation in intensity of black patterns was also continuous, and greatly influenced by temperature. Nymphs with intensive black patterns occurred at low temperatures and the incidence of such individuals decreased with an increase in temperature. Rearing density had only a moderate effect on the induction of black patterns: nymphs with more black patterns occurred at a higher density, but even under isolated conditions some nymphs developed intensive black patterns.