1. We have studied the cholinesterase activity from the skeletal muscle of the ammocoete of the lamprey Petromyzon marinus. 2. On the basis of pharmacologic and kinetic criteria, we conclude that the enzyme is true acetylcholinesterase, not pseudocholinesterase. 3. The acetylcholinesterase was found to be present in both globular and asymmetric forms. 4. In contrast to muscle of adult spawning lamprey, where globular esterase is almost exclusively G4, we found that muscle from ammocoete also contains significant amounts of G1 and G2. 5. This difference may be related to the physiological states of the lamprey during the various stages of its life cycle.