Abstract The pharynx protractor muscle of holothuria Cucumaria japonica responds by contraction to both nicotinomimetics and muscarinomimetics. The assumption that it is due to the existence of the two kinds of cholinoreceptors on the muscle fibres has been tested. The following findings support the assumption: 1. 1. Partial agonists-nicotinomimetics block the response to full agonists-nicotinomimetics, and do not block the response to muscarinomimetics. 2. 2. Benzilylcholine mustards and dibenamine antagonize the action of muscarinomimetics selectively without changing response to nicotinomimetics. 3. 3. N-cyclohexyl- N′-( β-4-methylmorpholinium)-ethyl-carbodiimide blocks selectively the response to nicotinomimetics, that to muscarinomimetics being unaffected. The preliminary experiments on the dentis retractor muscle of the sea urchin Stronglyocentrotus intermedius suggest that there are two kinds of cholinoreceptors, nicotinic and muscarinic, on this muscle too.