1. We report the cloning and expression of a novel Caenorhabditis elegans polypeptide, GLC-3, with high sequence identity to previously cloned L-glutamate-gated chloride channel subunits from nematodes and insects. 2. Expression of glc-3 cRNA in XENOPUS oocytes resulted in the formation of homo-oligomeric L-glutamate-gated chloride channels with robust and rapidly desensitizing currents, an EC(50) of 1.9+/-0.03 mM and a Hill coefficient of 1.5+/-0.1. GABA, glycine, histamine and NMDA all failed to activate the GLC-3 homo-oligomer at concentrations of 1 mM. The anthelminthic, ivermectin, directly and irreversibly activated the L-glutamate-gated channel with an EC(50) of 0.4+/-0.02 microM. 3. The GLC-3 channels were selective for chloride ions, as shown by the shift in the reversal potential for L-glutamate-gated currents after the reduction of external Cl(-) from 107.6 to 62.5 mM. 4. Picrotoxinin failed to inhibit L-glutamate agonist responses at concentrations up to 1 mM. The polycyclic dinitrile, 3,3-bis-trifluoromethyl-bicyclo[2,2,1]heptane-2,2-dicarbonitrile (BIDN), completely blocked L-glutamate-induced chloride currents recorded from oocytes expressing GLC-3 with an IC(50) of 0.2+/-0.07 microM. The phenylpyrazole insecticide, fipronil, reversibly inhibited L-glutamate-gated currents recorded from the GLC-3 receptor with an IC(50) of 11.5+/-0.11 microM. 5. In this study, we detail the unusual antagonist pharmacology of a new GluCl subunit from C. elegans. Unlike all other native and recombinant nematode GluCl reported to date, the GLC-3 receptor is insensitive to picrotoxinin, but is sensitive to two other channel blockers, BIDN and fipronil. Further study of this receptor may provide insights into the molecular basis of non-competitive antagonism by these compounds.