Soluble cholinesterases were purified from the cnidarians, Velella velella (Hydrozoa), and Actinia equina (Anthozoa), using 100,000g supernatants of total homogenates by affinity chromatography on a procainamide-containing gel. An overall purification of 2,340- and 1,970-fold, respectively, was achieved with an about 10% final yield. Both purified enzymes (relative molecular mass: 300 kDa, V. velella; 330 kDa, A. equina) seem to be tetrameric proteins consisting of four apparently identical or quite similar monomers. According to the values of kinetic constants, kcat and kcat/Km values, both enzymes are acetylcholinesterases. However, on the basis of kinetic and inhibition studies, the cholinesterase from A. equina shows a poor substrate affinity and an approximately 20-fold lower catalytic efficiency in comparison with the enzyme from V. velella. These findings suggest in the former a less specific and relatively unspecialized conformation of the active site. Such different features are likely due to particular adaptive patterns of cnidarians, also involving their nervous system. © 1992 Wiley-Liss, Inc.