Evidence is presented for an XX/XY sex-determining mechanism in five species of the genus Uta. All species examined have a somatic diploid complement of 12 macrochromosomes and 22 microchromosomes. The Y chromosome is morphologically distinguishable as the smallest microchromosome in the males. It is paired in metaphase I figures with an X chromosome which is two to three times its size. The size of the X chromosome does not distinguish it from other small microchromosomes in the karyotype.