A captive adult crevice kelpfish, Gibbonsia montereyensis, developed a cutaneous mass, approximately 9 × 7 mm on the right side of the head in an area of nonscaled skin. Following surgical debulking, examination of both impression smears and histologic sections of the tumor revealed a predominant population of round to spindloid to polygonal cells with a moderate amount of lightly basophilic cytoplasm. The cytoplasm was filled with round, variably-sized reddish-brown granules that often obscured the nucleus. Nuclei were round to ovoid with coarsely granular chromatin. There was minimal anisocytosis and anisokaryosis. The cytoplasmic granules in histologic sections were weakly positive by the Fontana-Masson method, and staining was eliminated with melanin bleach. Immunohistochemical staining was strongly positive with a murine monoclonal antibody for melan A. As the specificity of melan A for melanophores is not clearly defined in nonmammalian species, the tumor was examined by transmission electron microscopy. Melanophores were not detected. Instead, neoplastic cells were filled with numerous intracytoplasmic organelles with triple-limiting membranes composed of concentric lamellae; these structures were most compatible with pterinosomes, which are the pigment-containing organelles of cells called xanthophores and erythrophores. As both of these organelles are ultrastructurally indistinguishable and as kelpfish skin is known to contain both xanthophores and erythrophores, a diagnosis of a mixed pigment cell tumor or chromatophoroma was made. As the tumor was grossly reddish-brown, the possibility of a neoplastic population of only erythrophores could not be excluded. Pigment cell tumors, arising from cells of the embryonic neural crest, are common in reptiles and bony fish.