The oxygen consumption of an active tropical elasmobranch, the lemon shark (Negaprion brevirostris), was measured at different swimming speeds in an annular respirometer. The resulting power-performance curve had a slope of 0.36 which is similar to that of moderately active teleosts. The standard metabolic rate of this elasmobranch (95 ml O2.kg-1.h-1 was higher than that of other sharks, whereas the routine metabolic rate (110 ml O2.kg-1.h-1) was less than that of other elasmobranchs previously studied. Active metabolic rate (226 ml O2.kg-1.h-1), measured in one individual, was the highest for any elasmobranch measured to date. The cost of living of the tropical water lemon shark was calculated to be approximately 2.5 times greater than that of a temperate water shark, but not as high as a salmonid. It is possible that previous generalizations on the poor aerobic performance of sharks are based on data from the less active, temperate water sharks, and are misleading since the metabolic rate of this more active species compares well with active teleosts.