The energetics of locomotion depend largely on speed, gait and body size. Gait selection for a given speed appears partly, but perhaps not wholly, related to metabolic cost. One cost normally omitted from considerations of locomotion efficiency is the metabolic cost of the transition between gaits. We present the first direct assessment of the metabolic cost for the walk-run/run-walk transition in humans. The average increase in metabolic cost for a step involving a transition is 1.75 times that of a mean non-transition step at a speed where metabolic power requirements are identical for walking and running. Despite this substantial increase in cost for the transition step, the metabolic cost of gait transition is unlikely to have a strong bearing on the process of gait selection as the cost of using a metabolically inappropriate gait, even for only a few steps, will dominate.