The body-size scaling pattern of enzymes that are important in energy metabolism was examined in gills of the blue crab as a function of acclimation salinity. We hypothesized that the higher surface-area-to-volume ratio of small crabs would impose a greater metabolic cost for hyperosmoregulation, leading to an increase in the capacity for ATP production in gills. Postmetamorphic crabs spanning a 2,500-fold range in body mass were examined following a 7-d exposure to a salinity of 35, 17, or 5 ppt. The posterior gills, which are the principal site of osmoregulatory ion pumping, generally had higher activities than the anterior gills, which are primarily used for gas exchange, and this discrepancy was greatest in small crabs. A significant effect of salinity was found only for the enzyme citrate synthase, where the activity was highest at the lowest salinity. Although most enzymes scaled negatively with body mass, the activity was independent of size over a 250-fold size range that encompassed the body masses of juvenile crabs but decreased abruptly in the adult crabs. These data suggest that ion pumping associated with osmoregulation may represent a greater energetic challenge in smaller crabs, and this is reflected in the relatively higher metabolic potential of the posterior gills. However, acclimation to different salinity regimes does not lead to dramatic global changes in the capacity for energy metabolism.