1. Voluntary locomotion of the tortoise Testudo graeca was investigated over a range of experimental conditions. 2. The swing and stance intervals of the step cycles of individual limbs and the interlimb intervals were shown to depend only upon the locomotor cycle duration independent of experimental conditions. 3. The gait is described by four simple equations, valid for all animals and conditions tested, with locomotor cycle duration as the only independent variable. 4. The observed frequency of locomotion was shown to be related to body weight, cloacal temperature, and walking surface ina manner which can be described by a single equation. 5. The implications of these findings for vertebrate locomotion are discussed.