1. The activity of choline acetyltransferase (ChAT) has been determined in different parts of the swimbladder of two physostome species, the rainbow trout,Salmo gairdneri, and the eel,Anguilla anguilla, and three physoclist species, the Atlantic cod,Gadus morhua, the perch,Perca fluviatilis and the goldsinny wrasse,Ctenolabrus rupestris. 2. High activities of ChAT were found in the gas gland of the physoclist species. Preganglionic denervation (in the cod) only partly reduces this activity. It is concluded that the gas gland has a postganglionic cholinergic innervation. 3. The secretory mucosa outside the gas gland shows intermediate ChAT activity in the wrasse and low activity in the cod and the perch. 4. Very low activities of ChAT were found in the oval edge of the cod and the perch, indicating little if any cholinergic innervation of this area. The resorbent area of the wrasse swimbladder shows an intermediate activity. 5. In the eel the secretory mucosa shows low ChAT activity, indicating sparse innervation of the secretory cells. The resorbent mucosa contains a much higher activity. 6. The rainbow trout has an evenly distributed intermediate ChAT activity in the swimbladder.