We observed color changes of the katydid, Conocephalus maculatus, from the hatchling to the adult stage under various rearing conditions using two populations in southern Osaka Prefecture. C. maculatus showed color polymorphism in the nymphal stage as well as in the adult stage: Although all the hatchlings were green irrespective of populations or conditions, the rate of green morph (G rate) decreased to 40‐80% in the adult stage. Moreover, about 10% of individuals which had once turned brown returned to the green morph after one of the succeeding moults. The G rate of progenies between a green female and a green male tended to be higher than that between a brown female and a brown male throughout the nymphal stage, while environmental factors such as temperature, humidity and background color had no significant effect on nymphal color morph. The present results show that color-morph determination is controlled mainly by genetic factors and little affected by environmental factors in nymphs and adults of C. maculatus.