Using acetylthiocholine as substrate, microscopically localizable cholinesterase (ChE) activity is demonstrated in neural and glial elements of central and peripheral nervous systems of the lobsters, Panulirus argus and Homarus americanus. Moderate to very intense ChE activity occurs in all synaptic regions of the central ganglia and stomatogastric ganglion, in glial sheaths around neuron somata and peripheral nerve axons, and in cytoplasm of a few nerve cell bodies. Axons, identified as motor, contain extremely little ChE. The principal reaction in peripheral nerves occurs in sheath elements of sensory fibres; in most cases, much of the reaction is lost as the nerves lose the sheaths at the point of entry into brain.