Chromosome analysis was made on somatic cells in culture derived by necropsy from the ear and the peritoneum of a sex-chromatin positive male tortoiseshell domestic cat. A mixture of triploid (57, XXY) and normal diploid female (38 XX) cells was found in cultures of both tissue origins. The chimerism † probably existed in the cat and did not arise during in vitro cultivation. Several mechanisms for the origin of this particular chimera are suggested, including double fertilization and fusion of various types of fertilized or unfertilized meiotic products. It is postulated that the sex-linked allelic genes for the black and yellow coat color in cats follow Mendelian inheritance and that the tortoiseshell phenotype possibly is due to genetic inactivation of either X-chromosome in a heterozygous female. It is further suggested that a male tortoiseshell cat can possess an XXY sex chromosome constitution and be heterozygous for the sex-linked coat-color genes, but other types of sex chromosome anomalies and mosaicism can give rise to the same phenotype in males. The karyotype of the normal domestic cat (Felts domestica) is presented. Most, if not all, of the chromosomes can be identified. Certain similarities of karyotypes among members of Felidae are note