A new genetic trait, axanthic, is described in the Mexican axolotl with a phenotype completely lacking visible xanthophores and iridophores. Animals received from the colony of Dr. L. E. DeLanney at Ithaca, New York, have been mated among themselves for a total of seven spawnings, in which the axanthic trait was inherited as a simple Mendelian recessive. The symbol for this newly found gene is ax. Homozygous recessive animals appear normal except for their lack of visible xanthophores and iridophores. The results of the spawnings also indicate that the axanthic gene is not linked to the melanoid gene ( m). Its linkage with other known mutants in the axolotl is being tested. Hanging-drop preparations in Niu-Twitty's solution of neural fold explants were made using axanthic and normal embryos. Xanthogenesis was never visible in cultures from axanthic animals, whereas all those made from yellow animals had fully differentiated xanthophores within 9 days after the explants were made. Counts of cells in cultures from axanthic animals showed a complement of axanthic xanthoblasts, that is, otherwise normal xanthophores without pigment. Epidermal vesicles were made by combining flank somatopleure from early tailbud embryos with neural fold from neurulae. Axanthic somatopleure was combined with axanthic and xanthic neural fold (in separate vesicles) and xanthic somatopleure combined with folds from axanthic embryos. Only the vesicles with neural fold from xanthic donors had visible xanthophores. Homotopic reciprocal neural crest transplants were made between axanthic and yellow larvae at midtailbud stages. The grafted regions were obvious after pigment synthesis had begun. Xanthophores were visible in these regions whenever the donor material was derived from a yellow larva. The graft region was devoid of visible xanthophores whenever the donor material was axanthic neural crest, even when the crest was transplanted onto xanthic animals. Chromatographic analyses of the pteridine contents in axanthic as compared to normal animals showed that the axanthic animals were lacking five fluorescent spots found in normal larvae. Two of these spots from normal animals had yellow fluorescence; three had blue fluorescence. The three blue fluorescent compounds were tentatively identified as Ranachrome-3, Hynobius-blue, and 2-amino-4-hydroxypteridine-6-car☐ylic acid. One yellow spot appeared to be sepiapterin. The other could not be identified. The chromatographic results indicate that the axanthic mutant is deficient in these pteridine compounds, which are evidently necessary for the pigmentation of xanthophores. The results from the experimental work offer support to the hypothesis that the axanthic trait, inherited as a simple Mendelian recessive, acts at the level of the xanthoblasts to prevent the accumulation of yellow pigments in these cells. As there are no visible iridophores in these mutants either, it may also prevent the accumulation of pigmentary purines in iridoblasts.