Membrane acetylcholinesterase activity is considered to be a marker for a cholinergic system. When temporarily expressed in differentiating cells other than the nervous or muscular ones, it may play a role in morphogenesis. In the lichen Parmelia caperata (L.) Ach., acetylcholinesterase is histochemically localized mainly in the cell walls and/or membranes of both symbionts just where they proliferate and form well-organized propagation structures, the soredia. The enzyme activity is first detected in a few algae undergoing aplanosporogenesis and later in medullary hyphae that reach the dividing algae by elongating perpendicularly to the thallus surface. This histochemical pattern that is associated with algal proliferation and oriented hyphal growth is characteristic of early morphogenesis of the soredia; when fully differentiated, they consist of an inner dividing alga and an outer hyphal envelope, both showing cholinesterase activity. Substrate specificity and inhibitor sensitivity of the histochemical staining indicate an acetylcholinesterase-like activity. However, extracts of the thallus areas where soredia develop give four bands of cholinesterase activity on disc electrophoresis: the two cathodal bands have the characteristics of acetylcholinesterase, the others of pseudocholinesterase. One of the latter hydrolyses propionylthiocholine very rapidly. The findings suggest that in lichen symbiosis, a cholinergic-like system participates in regulating morphogenetic processes such as cell division, oriented tip growth and alga-fungus membrane interactions. Environmental stimuli, particularly light, might trigger the development of soredia by modulating the activity of the cholinergic mechanism.