It is well established that the metabolic cost of horizontal locomotion decreases as a regular function of animal body mass, regardless of body form and phylogeny. How body size affects the cost of incline exercise remains much less clear. Studies on vertebrates have led to the hypotheses that the cost of vertical work is independent of body mass and that the added cost of locomoting on inclines is lower for small animals. Studies on vertebrates and a few invertebrates provide evidence both for and against these hypotheses. To gain further insight into the cost of incline exercise, we measured oxygen consumption of small (2.33 ± 0.07 g) and large (46.66 ± 5.33 g) ghost crabs (Ocypode quadrata) locomoting horizontally and up a 20° incline. The slope of the oxygen consumption versus speed relationship (= minimum cost of transport) was not significantly different for small crabs exercising horizontally and on an incline. However, the intercept for incline exercise was significantly higher, indicating that small crabs used more energy during incline exercise than during horizontal exercise. Incline had no effect on the slope or intercept of the oxygen consumption versus speed relationship for large crabs. Our results suggest that the cost of incline locomotion may be large for small animals and that the cost is not independent of body size. Our results add to the growing body of research indicating that body mass is but one factor that determines the cost of incline locomotion and efficiency of vertical work.