Abstract 1. Soluble cholinesterases were purified from the crustaceans Maia verrucosa and Palinurus vulgaris, belonging to the same order (Decapoda) and suborder (Reptantia). 2. Purification was carried out from 100,000 g supernatants by affinity chromatography on a procainamide-containing gel. 3. Each purified enzyme is a single protein giving differently sized subunits on PAGE-SDS electrophoresis. 4. According to k cat/ K m values, the enzymes are propionylcholinesterase ( M. verrucosa) and butyryl-cholinesterase ( P. vulgaris), even if acetylthiocholine and propionylthiocholine respectively display the highest rates of hydrolysis. 5. According to kinetic constants, the enzyme from M. verrucosa mainly discriminates among substrates (choline or p-nitrophenol esters) on the basis of steric hindrance and hydrophobic interactions. Catalytic efficiency of the enzyme from P. vulgaris is mostly affected by electrostatic forces. Charge-bearing substrates are quite better for both the cholinesterases. 6. Kinetic properties of the studied enzymes are compared with those of other Invertebrata and their possible phylogenetic and adaptive features are discussed.