Acetylcholinesterase, an essential enzyme of the nervous system, rapidly terminates the action of acetylcholine released into the synapse. Acetylcholinesterase is also found (in lower abundance) in extrajunctional areas of muscle and nerve and on erythrocyte membranes^1,2. Hydrodynamic analyses of the native enzyme³ and characterization of its dissociated subunits have revealed multiple enzyme forms which can be divided into two classes: (1) dimension-ally asymmetric forms which are usually found within the synapse and contain a collagen-like structural subunit disulphide-linked to the catalytic subunits; and (2) globular forms which appear to be widely distributed on the outer surface of cell membranes. Both forms have been characterized in the ray Torpedo californica and, although their catalytic behaviours seem to be identical, they differ slightly in amino-acid composition, peptide maps and reactivity with certain monoclonal antibodies^4–7 . Here, we report the complete amino-acid sequence of an acetylcholinesterase inferred from the sequence of a complementary DNA clone. The 575-residue protein shows significant homology with the C-terminal portion of thyroglobulin⁸.