Abstract 1. 1. Specific activity and kinetic characteristics of the (Na + + K +)ATPase have been investigated in the gill epithelium of the hyper-hypoosmoregulator crab Uca minax. 2. 2. (Na + +K +)ATPase activity is shown to be at least three times higher in the posterior gills. 3. 3. The kinetic study supports the hypothesis of the existence of two different (Na + + K +)ATPases: the enzyme activity in the posterior gills could be involved in the transepithelial transport of Na + while the activity of the anterior gills could be responsible for the intracellular regulation of Na + and K +. 4. 4. Significant and specific changes in (Na + +K +)ATPase activity occur upon acclimation to media of various salinities.