The article presents the first 3D structure of an inhibitory anion-selective Cys-loop receptor, the homopentameric *Caenorhabditis elegans* glutamate-gated chloride channel α (GluCl), at 3.3 Å resolution. The structure was determined using X-ray crystallography with the allosteric agonist ivermectin and additional structures with the endogenous neurotransmitter L-glutamate and the open-channel blocker picrotoxin. Ivermectin binds in the transmembrane domain of the receptor, stabilizing an open pore conformation. Glutamate binds in the classical agonist site at subunit interfaces, and picrotoxin directly occludes the pore near its cytosolic base. The structure provides insights into the mechanisms of fast inhibitory neurotransmission and allosteric modulation of Cys-loop receptors.The article presents the first 3D structure of an inhibitory anion-selective Cys-loop receptor, the homopentameric *Caenorhabditis elegans* glutamate-gated chloride channel α (GluCl), at 3.3 Å resolution. The structure was determined using X-ray crystallography with the allosteric agonist ivermectin and additional structures with the endogenous neurotransmitter L-glutamate and the open-channel blocker picrotoxin. Ivermectin binds in the transmembrane domain of the receptor, stabilizing an open pore conformation. Glutamate binds in the classical agonist site at subunit interfaces, and picrotoxin directly occludes the pore near its cytosolic base. The structure provides insights into the mechanisms of fast inhibitory neurotransmission and allosteric modulation of Cys-loop receptors.