1. 1. The effects of zinc ions (Zn 2+) on the activities of Mg 2+-, Ca 2+-, Na +-, K +-, and Na +NH 4 +-ATPase in preparations of branchial microsomes from the gills of freshwater acclimated rainbow trout were studied. 2. 2. Inhibition of all ATPases occurred from 1 to 100 mg·1 −1 Zn 2+ for assays conducted at 11°C. Na +NH 4 +-ATPase was the most sensitive to inhibition by Zn 2+ followed by Na +K +-ATPase while Mg 2+- and Ca 2+-ATPase were the least inhibited by comparable concentations of the metal. 3. 3. The effects of Zn 2+ in 37°C assays paralleled those conducted at 11°C with the exception that Ca 2+-ATPase was stimulated by 15, 50 and 100 mg·1 −1 Zn 2+. 4. 4. Zinc ions hydrolyzed adenosine triphosphate (ATP) in both 11 and 37°C incubations with maximum specific activity obtained at 150 mg·1 −1. 5. 5. Zinc may be able to replace divalent cations as cofactors or substrates thus accounting for the observed ATP hydrolysis. 6. 6. The response of gill ATPases to Zn 2+ at 37 and 11°C were not similar in all cases and demonstrate the need to perform enzyme assays at or near the preferred temperature of the fish.