Electromyography and cinematography were used to determine the activity of epaxial muscles of colubrid snakes during terrestrial and aquatic lateral undulatory locomotion. In both types of lateral undulation, at a given longitudinal position, segments of three muscles (Mm. semispinalis-spinalis, longissimus dorsi, and iliocostalis) usually show synchronous activity. Muscle activity propagates posteriorly and generally is unilateral. With each muscle, large numbers of adjacent segments (30 to 100) show simultaneous activity. Terrestrial and aquatic undulation differ in two major respects. (1) During terrestrial undulation, muscle activity in a particular region begins when that portion of the body has reached maximal convex flexion and ends when it is maximally concave; this phase relation is uniform along the entire snake. During swimming, however, muscle activity passes posteriorly faster than the wave of vertebral flexion, causing the relation of muscle activity to flexion to change along the length of the snake. (2) In the terrestrial mode, the block of active muscle segments remains approximately constant in size as it passes down the snake, whereas during swimming the number of adjacent active muscle segments increases posteriorly. Despite the fact that Elaphe obsoleta has nearly twice as many body vertebrate as Nerodia fasciata (240 vs. 125), the only difference observed in the swimming of these two species is that a larger number of adjacent muscle segments is simultaneously active in comparable regions of Elaphe obsoleta than in Nerodia fasciata.