Nicotinic acetylcholine (ACh) receptors (nAChRs) are important excitatory neurotransmitter receptors in the insect CNS. We have isolated and characterized the gene and the cDNA of a new nAChR subunit from Drosophila. The predicted mature nAChR protein consists of 773 amino acid residues and has the structural features of an ACh-binding alpha subunit. It was therefore named D alpha3, for Drosophila alpha-subunit 3. The d alpha3 gene maps to the X chromosome at position 7E. The properties of the D alpha3 protein were assessed by expression in Xenopus oocytes. D alpha3 did not form functional receptors on its own or in combination with any Drosophila beta-type nAChR subunit. Nondesensitizing ACh-evoked inward currents were observed when D alpha3 was coexpressed with the chick beta2 subunit. Half-maximal responses were at approximately 0.15 microM ACh with a Hill coefficient of approximately 1.5. The snake venom component alpha-bungarotoxin (100 nM) efficiently but reversibly blocked D alpha3/beta2 receptors, suggesting that D alpha3 may be a component of one of the previously described two classes of toxin binding sites in the Drosophila CNS.