Abstract Acetylcholinesterase and butyrylcholinesterase occur under several molecular forms in vertebrate tissues. In the asymmetric, low-salt aggregating forms, the catalytic subunits are linked to a collagen-like ‘tail’. These molecules appear to be associated with extracellular basal lamina, e.g. in neuromuscular junction. Other tail-less, or globular, forms which are predominant in most tissues of higher vertebrates. include membrane-bound as well as soluble variants. There are clear structural homologies between the various forms of these two enzymes throughout the range of vertebrates. Such a polymorphism probably ensures an optimal functioning of various cholinergic structures.