Several taxa of marine fishes are capable of both aquatic and terrestrial locomotion, yet the terrestrial locomotion mechanics of many of these taxa have not been studied. Many species of the prickleback family Stichaeidae, including the Rock Prickleback, Xiphister mucosus, are capable of both aquatic and terrestrial locomotion. In this study, high-speed video was used to compare the mechanics of aquatic and terrestrial locomotion of X. mucosus. While swimming, tail-beats occur faster, and the velocities of the head and tail are greater than on land. On land, tail-beats are slower and cover a longer distance, yet the distance traveled by the head is similar to the distance traveled during aquatic locomotion. Froude propulsion efficiencies during swimming average 0.75 (s  =  0.05) indicating efficient swimming mechanics in X. mucosus. These efficiency values are typical for other anguilliform swimmers that make terrestrial excursions. Overall, X. mucosus makes slight modifications to its efficient swimming locomotion mechanics, principally by increasing movement of the tail, to move effectively over the terrestrial environment.