Use of the aquatic environment by hippopotami (Hippopotamus amphibius) allows locomotive styles impossible to achieve on land by such heavy animals. Videos of the underwater locomotion of 2 hippopotami were analyzed frame by frame. Average horizontal velocity underwater was 0.47 m/s. Hippopotami used a gait underwater that was similar to a gallop with extended unsupported intervals. Ground contact time decreased with increasing horizontal velocity, vertical displacement during the unsupported intervals increased with an increase in ground contact time, and time between consecutive footfalls decreased with an increase in horizontal velocity. Hippopotami use an unstable gait underwater, which is facilitated by the increased buoyancy of water. Despite restrictions to movement on land due to its massive weight, locomotion of the hippopotamus underwater is analogous to movement in a microgravity environment.