Pharmacological experiments on Ascaris suum have demonstrated the presence of three (N-, L-, and B-) subtypes of cholinergic receptor mediating contraction of body wall muscle in parasitic nematodes (1). In the present study, these ionotropic acetylcholine (Ach) receptors (nAChRs) were activated by levamisole and bephenium under patch-clamp conditions and competitively antagonized by paraherquamide and 2-desoxoparaherquamide. A number of recordings exhibited three separate current amplitude levels, indicating the presence of small, intermediate, and large conductance subtypes of receptor. The mean conductance of the small conductance subtype, G₂₅, was 22 ± 1 pS; the intermediate conductance channel, G₃₅, was 33 ± 1 pS; and the large conductance channel, G₄₅, was 45 ± 1 pS. The small channel was not antagonized significantly by paraherquamide and was identified as the N-subtype. The intermediate channel was preferentially activated by levamisole rather than bephenium and antagonized by paraherquamide: the intermediate channel was identified as the L-subtype. The large conductance channel was preferentially activated by bephenium, antagonized more by 2-desoxoparaherquamde than by paraherquamide and was identified as the B-subtype. These observations reveal that the three channel subtypes have different selectivity for cholinergic anthelmintics. The different selectivity of these compounds should be considered when dealing with drug resistant infections.--Qian, H., Martin, R. J., and Robertson, A. P. Pharmacology of N-, L-, and B-subtypes of nematode nAChR resolved at the single-chain level in Ascaris Suum.