Generally, male and female heterogamety of amphibians and reptiles cannot be determined cytologically, as the X and Y or the Z and W appear morphologically identical. However, female heterogamety of the African water frog, Xenopus laevis, has been proven by the sex reversal experiments of Chang and Witschi. The present report describes the cytological identification of the sex chromosomes in this species. The Z-chromosome is the smallest element in the complement with a subterminal kinetochore, while the submediocentric W exceeds the largest autosome in size. Thus a distinct heteromorphism exists between the Z and W in this primitive anuran.