Proteolytic fragmentation of [3H]diisopropylfluorophosphate-labelled catalytic subunits of different molecular forms of acetylcholinesterase demonstrates that all forms extracted from the electric organ from Torpedo marmorata are true acetylcholinesterases. This is supported by immunochemical results showing that the radiolabelled polypeptides are readily recognized by specific anti-acetylcholinesterase antibodies. Although distinct structural differences exist, all forms contain a similar peptide carrying the serine hydroxyl of the esteratic subsite. Dimeric, detergent-soluble acetylcholinesterase is present in the low-salt-soluble extract (Mr of the catalytic subunit 66,000) together with a monomeric form (apparent Mr 76,000). This monomeric polypeptide is hydrophilic, enzymatically inactive, and might represent a precursor of the asymmetric forms of acetylcholinesterase.