We report the presence of pigmented skin tumors in three populations of the endangered amphibian Montseny brook newt, Calotriton arnoldi, one of the European amphibian species with the smallest distribution range (40 km(2) in the Montseny Natural Park, Catalonia, Spain). Examination of one of the tumors by light microscopy was consistent with chromatophoroma and was most suggestive of a melanophoroma. Tumors were not found in juveniles. In adults, only two of three populations were affected. The proportions of males and females affected were not significantly different, but there was a positive correlation between body size and presence of tumors in both sexes. The etiology of chromatophoromas remains unknown but, in our study, they do not appear to have been caused by water quality or Ultraviolet B.