The ultrastructure of the black epidermal hooks occurring on the forelimbs and fingers of Xenopus has revealed a very strong keratinization of surface cells. Specific staining with silver-methenamine permitted the identification of keratin components and of melanin in dermal melanophores. As no evidence of cytocrine pigmentation could be located in the epiderm, the colouring of the hooks was attributed to the markedly different structure of these cells