Two cDNAs (Ce21 and Ce13) were isolated from a Caenorhabditis elegans library screened with a probe encoding conserved domains of the avian alpha5 neuronal nicotinic acetylcholine receptor (nAChR). Alignments to all nAChR subunits in the EMBL/Swissprot data base demonstrate that the Ce21 protein most resembles the vertebrate alpha7 subunit, whereas Ce13 is closest to the ARD subunit of Drosophila. The corresponding genes were isolated and hybridized to YAC grids: Ce21 maps on chromosome V near the his-23 gene, and Ce13 on chromosome I very near or at unc-29. The structure of the Ce21 gene was compared with that of other vertebrate and invertebrate nAChR genes and found to share by far the largest number of conserved splice sites with the vertebrate alpha7 gene. Upon expression in the Xenopus oocyte system, the Ce21 subunit assembled into a functional homomeric nAChR, whose properties were compared with those of the chicken alpha7 receptor. The anthelmintic nicotinic agonist levamisole is unable to activate the Ce21 and alpha7 receptors, but efficiently antagonises their responses to ACh. Both receptors desensitise quickly upon agonist application, are more sensitive to nicotine than to acetylcholine, and are efficiently blocked by dihydro-beta-erythroidine. Unlike the alpha7 receptor, however, the Ce21 receptor is relatively insensitive to methyllycaconitine and to alpha-bungarotoxin. The similarities in protein sequence, gene structure and physiological properties between alpha7 and Ce21 suggest a very ancient lineage for the alpha7 class of nAChR subunits.