Diversity of nicotinic acetylcholine receptors (nAChRs) mediating Cl-current in voltage-clamped identifiable Lymnaea stagnalis neurons was studied using acetylcholine (ACh), three agonists and α-conotoxin ImI (ImI). Cytisine, nicotine, and choline, full agonists at α7 subunit-containing nAChRs of vertebrates, were found to evoke at saturating concentration 84-92% of the maximal current elicited by ACh. ImI, known to block selectively α7 and α9 nAChRs, markedly diminished the responses to ACh. The average maximal ImI-induced block was 80%, leaving a residual current which had very slow kinetics. The choline-, cytisine-, and nicotine-induced currents were blocked by ImI almost completely, suggesting that they activate only ImI-sensitive receptors. Two groups of cells which differ in desensitization kinetics and in sensitivity to ImI were revealed. IC50values for ImI against ACh were 10.3 and 288 nM, respectively, with the rapidly desensitizing current being the more sensitive to ImI. The data obtained suggest the existence of at least three pharmacologically distinct subtypes of nicotinic receptors in Lymnaea neurons. Two of the subtypes are similar to α7 nAChRs of vertebrates, but differ from each other in their affinity for ImI and in their desensitization kinetics. The third subtype is quite distinct, in that it is resistant to ImI, is not activated by nicotine, cytisine or choline, and mediates a very slowly developing current. © 2004 Elsevier Ireland Ltd. All rights reserved.