Melanosomes observed in dermal melanophores of adult leaf frogs contain a unique wine red pigment identified as pterorhodin, a pteridine dimer never before found in any vertebrate. This type of melanosome, almost twice as large as the typical eumelanin melanosome, contains a small electron dense core of eumelanin surrounded by a concentric fibrous mass of pterorhodin. Dermal melanophores of larval leaf frogs contain small eumelanin melanosomes that transform at metamorphosis through the gradual accumulation of pterorhodin on the eumelanin surface to form the compound melanosomes of adults. This process may be mediated by thyroxine. No explanation for the unique presence of pterorhodin in leaf frogs has yet surfaced. A variety of tree frog species from Australia and Papua New Guinea also possess pterorhodin and the large melanosome suggesting that tree frogs from the New World and those from Australia are closely related and may have been separated during continental drift. Several of the unsolved problems posed by the emergence of pterorhodin in a unique melanosome are discussed.