Male sand fiddler crabs, Uca pugilator,permitted to enter or leave sea water (SW) at will (natural environment) hyperosmoregulated in 10% and 50% SW and hypoosmoregulated in 150% and 200% SW. Crabs forced to remain submerged in the SW media (immersion) did not osmoregulate as well, particularly in 150% and 200% SW. Gill sodium-plus-potassium-activated ATPase (Na,K-ATPase) specific activity was characterized in crude homogenates, and optima for K, Mg, ATP and pH were determined. After 21 days acclimation in a 10% SW natural environment, gills 1, 2, 5, and 6 showed significant increases in gill enzyme-specific activity (ESA) compared to 100% SW controls. Acclimation to 50% SW caused significant increases in ESA only in gills 5 and 6; acclimation to 150% and 200% SW natural environments decreased or did not significantly change ESA. Gills 5 and 6 contained approximately 75% of total gill ESA in all media. Crabs immersed in the SW media showed similar changes in gill ESA. It appears that changes in ESA in the posterior gills are important only in hyperosmotic regulation and in active Na uptake in dilute media. On transfer of 100% SW acclimated crabs to 10% SW (immersion), gill crude-homogenate ESA increased significantly after 3 days and reached maximal activity after 7 days. Gill microsomal preparations showed a slower time course; ESA did not increase significantly until 7 days after transfer and did not reach maximal levels until day 14 after transfer. These data are consistent with the hypothesis that increased gill ESA may be due to both the activation of existing enzyme and the synthesis of new enzyme by the gill epithelium in U. pugilator.