The energetics and hydrodynamics of surface and submerged swimming were compared in the sea otter (Enhydra lutris). 1. Sea otters used two distinct speed ranges that varied with swimming mode. Sustained surface swimming was limited to speeds less than 0.80 m/s, while sustained submerged swimming occurred over the range of 0.60 to 1.39 m/s. 2. Rates of oxygen consumption (VO2) at the transition speed (0.80 m/s) were 41% lower for submerged swimming by sea otters in comparison to surface swimming. 3. Total cost of transport for surface swimming sea otters, 12.56 joules/kg.m, was more than 12 times the predicted value for a similarly-sized salmonid fish. Transport costs for submerged swimming at the same speed was only 7.33 times the predicted value. 4. The allometric relationship for minimum cost of transport in surface swimming birds and mammals was y = 23.87 chi -0.15 where y = cost of transport in joules/kg.m and x = body mass in kg. This regression loosely parallels the relationship for salmonid fish. 5. Correlations between aquatic behavior, morphological specialization, and swimming energetics indicate that the development of swimming in mustelids involved transitions from fore-paw to hind-paw propulsion, and from surface to submerged swimming.