Ultrastructural localization techniques for cholinesterases (ChE) by the Karnovsky method, for monoamines by the technique of Wood, and Tranzer and Richards, and for 5HT by the technique of Wood, were performed on the anterior byssal retractor muscle of Mytilus. Simultaneous ultrastructural localization of 5HT and ChE was obtained by the use of the false neurotransmitter 5,6-DHT, followed by the method of Karnovsky for ChE. The glio-interstitial tissue presents a constant ChE activity, mainly localized on the plasma membrane, even in those processes which accompany tryptaminergic (5HT containing) neurites. Muscle cell membrane also reacts positively to the Karnovsky method. The tryptaminergic neurites themselves do not show any ChE activity; they contain dichromate reactive large (100 nm) dense cored vesicles. The presence of differentiated tryptaminergic neuromuscular junctions, suggested by other authors, is established. It was not possible to distinguish classes of nerve endings by the typology of their vesicular content. It is concluded that one can reasonably plan to study the effect of glial ChE inhibition on the physiology of the tryptaminergic (relaxing) response of the A.B.R.M. to nerve stimulation.