Neurotoxins of natural origin have proved to be of considerable value in the isolation and characterization of vertebrate muscle and neuronal nicotinic acetylcholine receptors (nAChRs). To date, they have been used less extensively in studies of invertebrate nAChRs. Here we examine how a variety of neurotoxins (the snake toxins alpha-bungarotoxin, alpha-BGT, and kappa-bungarotoxin, kappa-BGT, the molluscan toxin, neosurugatoxin, and the soft coral toxins, lophotoxin and bipinnatin-B) can be used to characterize nAChRs in an insect, Periplaneta americana, and in a parasitic nematode, Ascaris suum. The agonist profiles of these nAChRs are distinct, but the most striking differences are in the actions of antagonists. Whereas the insect nAChR is blocked by both alpha- and kappa-bungarotoxins, the nematode receptor is only blocked by kappa-BGT. Neosurugatoxin blocks nAChRs in both species, but the lophotoxins which block all nAChRs investigated to date are much less effective on the Ascaris muscle receptor.