Chelydrids (including snapping and big-headed turtles) are unusual among extant turtles in possessing long, robust tails. In other lineages of quadrupedal reptiles, long tails perform critical functions during both terrestrial and aquatic locomotion, and the tails of Common Snapping Turtles have been shown to help stabilize juveniles as they ascend terrestrial slopes. However, Common Snapping Turtles live primarily in aquatic habitats, and the function of the tail in these environments has not been examined. The first step to evaluating the role of the chelydrid tail in water is to evaluate its pattern of motion; therefore, we collected high-speed digital video of tail kinematics from juvenile Common Snapping Turtles (Chelydra serpentina) during aquatic walking. Common Snapping Turtles hold the tail off the substrate and move it as a nearly rigid strut during aquatic walking, cyclically flexing it side to side by 11–12° from the body midline. These motions occur one-quarter cycle out of phase with the motions of the limbs; thus, the timing of tail movements suggests that they are likely not a passive consequence of hind-limb retraction and are likely controlled by one (or a combination) of tail muscles, rather than ipsilateral hind-limb retractors. The potential for tail movements to contribute to aquatic thrust in Common Snapping Turtles is uncertain. However, Common Snapping Turtle tail movements resemble those of salamanders and lizards in many respects, suggesting that Common Snapping Turtles might retain primitive tetrapod or sauropsid features of tail motor control despite possessing a radically divergent body plan.