Sensilla are sense organs in insects, generally comprising of a group of cuticle or epi-dermal cells that appear as hairs or rod-shaped structures. Sensilla serve as the functional elements of sensory systems. The morphology and ultrastructure of sensilla on the antennae of mature male and female Japanese sawyer beetles, Monochamus alternatus Hope (Coleoptera: Cerambycidae), were examined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Five types of sensilla (sensilla basiconica, sensilla trichodea, sensilla digit-like, sensilla rod-like, and sensilla bottle-like) were observed on antennae of males, and four types of sensilla (sensilla basiconica, sensilla trichodea, sensilla bottle-like, and sensilla chaetica) were observed on antennae of females. Sensilla basiconica and sensilla trichodea were found to be abundant on the front elevation of the antennae. TEM micrographs of sensilla basiconica on the antennae of M. alternatus clearly showed that they had no pits in the cuticular region. Micrographs of longitudinal sections of sensilla trichodea showed them to be flat-tipped and smooth-surfaced pegs with pointed tips that are suggestive of a gustatory function. TEM micrographs of sensilla chaetica longitudinal sections showed dendrite branches and no cuticular pore; these sensilla may be involved in the perception of humidity, temperature, heat, and CO₂. These results are discussed in relation to the possible roles of the sensilla types in the host-locating behavior of M. alternatus.