Summary.1 A survey of the investigations, made earlier on the choline esterase content in invertebrates, is a preface to this paper.2 The acetylcholine hydrolysing effect for different marine invertebrates has been investigated.3 The acetylcholine hydrolysing effect is compared with the hydrolysis of two other choline esters, acetyl‐β‐methylcholine and benzoylcholine, which other authors have suggested using in order to estimate the specificity of choline esterase. The results show that the method is insufficient. Thus some species hardly split either of the two substrates mentioned; this suggests the possibility of the occurrence of an enzyme which is still more specific as regards acetylcholine.4 The properties of the choline esterases are discussed in the light of prese