In the tadpole of the tree frog Hyla arborea, the color of the dorsal skin was dark brown. Dermal melanophores, xanthophores, and iridophores were scattered randomly under the subepidermal collagen layer (SCL). After metamorphosis, the dorsal color of the animal changed to green and the animal acquired the ability of dramatic color change, demonstrating that the dermal chromatophore unit (DCU) was formed at metamorphosis. Fibroblasts invaded the SCL and divided it into two parts: the stratum spongiosum (SS) and the stratum compactum (SC). The activity of collagenase increased at metamorphosis. The fibroblasts appeared to dissolve the collagen matrix as they invaded the SCL. Then, three types of chromatophores migrated through the SCL and the DCU was formed in the SS. The mechanism how the three types of chromatophores were organized into a DCU is uncertain, but different migration rates of the three chromatophore types may be a factor that determines the position of the chromatophores in the DCU. Almost an equal number of each chromatophore type is necessary to form the DCUs. However, the number of dermal melanophores in the tadpoles was less than the number of xanthophores and iridophores. It was suggested that epidermal melanophores migrated to the dermis at metamorphosis and developed into dermal melanophores. This change may account for smaller number of dermal melanophores available to form the DCUs.