Male heterogamety, interpreted as an XX/XY sex-chromosome system, occurs in the turtle genus Staurotypus (family Kinosternidae). Meiotic analysis showed that the basis for the heteromorphism resides in relatively small regions of a pair of macrochromosomes. Heteromorphism was not observed in the three other genera belonging to the same family. Available data for reptiles and higher vertebrates strongly suggest that sex-chromosome heterogamety has evolved independently at least three times in reptiles (in turtles, lizards, and snakes) and, therefore, that the sex chromosomes of birds do not share a common ancestry with those of snakes.