The peculiar type of locomotion characteristic of the African species of Cerastes and the Amer. sidewinding rattlesnake, Crotalus cerastes, is analyzed and shown to be an essentially rolling movement. During this movement only 2 limited sections of the snake''s body are in contact with the ground, the other parts being lifted. This locomotion is similar to the rolling of a circular helix, which in fact leaves exactly the same tracks. Thus sidewinding is not a gliding or translatory movement on the substratum. It is not restricted to the known "sidewinders.".