In general, each insect olfactory sensory neuron expresses one member of the olfactory receptor (OR) gene family along with the highly conserved Or83b co-receptor, and the two ORs form a heteromeric complex to function as a chemosensor. Here we provide evidence that heteromeric insect ORs comprise a novel class of ligand-activated nonselective cation channels. Heterologous cells expressing silk moth, fruit fly, or mosquito heteromeric OR complexes exhibited extracellular calcium influx and cation-nonselective ion conductance upon odorant or pheromone stimulation. G protein-mediated signaling was negligible in producing the current elicited by ligand activation of insect ORs. Direct evidence for ligand-gated channels was obtained by outside-out patch-clamp single-channel recording of Xenopus oocyte and HEK293T cell membranes expressing insect OR complexes. The olfactory signal transduction mechanism in insects is clearly distinct from that in vertebrates and appears to be a unique strategy that insects have acquired to respond to the olfactory environment.