Xanthophores isolated from bullfrog tadpoles were successfully cloned. The rapid disappearance of the yellow pigment-containing organelles (pterinosomes) occurred in proliferating xanthophores and many black granules were newly synthesized in the cells. The granules were electronmicroscopically identified as melanin granules. These chromatophores showed melanin dispersion by the addition of the alkali-treated ACTH. The findings suggest that xanthophores transform into melanophores in vitro.