The multiporous plate sensillum occurs on the antennae of Hymenoptera. The internal and external cuticular surfaces of these sensilla, as well as the cellular ultrastructure, were examined in the Braconidae by electron microscopy. The sensillum has an elongated, external, porous plate and an inner, cuticular septum in which there is an aperture. It is innervated by numerous neurons, the dendrites of which pass through the aperture, branch and traverse the longitudinal axis of the sensillum. The ratio of aperture length to sensillar length was measured on the internal cuticular surface in 32 species. This ratio increased from the ancestral to the derived species. This sensillar character may be useful in elucidating braconid systematics. It also has potential use in the systematics of other hymenopterous parasitoids.