Substantially pure conotoxins are provided which inhibit synaptic transmissions at the neuromuscular junctions and which are useful both in vivo and in assays because they specifically target particular receptors, such as the acetyl-choline receptor, and ion channels. The peptides are of such length that they can be made by chemical synthesis. They also may be made using recombinant DNA techniques, and the DNA encoding such conotoxins having pesticidal properties can be incorporated as plant defense genes into plant species of interest.