Chemoreception is a key feature in selection of host plants by insects. We performed a preliminary functional characterization of olfactory proteins isolated from an antennal cDNA library of Monochamus alternatus. We identified four olfactory genes, including two encoding putative classic odorant-binding proteins (OBPs) and two encoding minus-C OBPs. We expressed two of the four OBPs, MaltOBP3 and MaltOBP5, in a bacterial system and assessed their ligand specificity by measuring the competitive binding of fluorescent probe, N-phenyl-1-naph-thylamine, in the presence of 17 volatile beetle- or host-plant-related ligands. The results indicated that although MaltOBP3 and MaltOBP5 bound a distinctly different group of competitors, both had relatively high binding affinities (Ki < 20 μm) for certain compounds. The differences in their binding affinities towards host-plant ligands suggest the roles of MaltOBP3 and MaltOBP5 in host-plant selection.