The sensillum capitulum, a hygro- and thermoreceptive sensillum of the cockroach Periplaneta americana L. is comprised of four receptor cells, one glial cell, two gland cells and four enveloping cells. Each receptor cell sends a single axon to the CNS, and has a single dendrite which extends distally. The perikarya of three receptor cells are enclosed by one of the enveloping cells (number one) and the fourth by a gland cell. The dendrites of all four receptor cells are enclosed by the same enveloping cell. The dendrites contain rootlets and many microtubules. The sensory cilia arise at the end of the dendrites at the level of the inner cavity, expand from the connecting cilium in a manner similar to those of the amphinematic scolopidia, and have numerous microtubules. The 1st enveloping cell contains scolopale rod-like structures. The sensory cilia are surrounded distally by a cuticular sheath formed within the 1st enveloping cell. One of the sensory cilia disappears distally, and the remaining ones enter the sensillum cone. Of these, one branches and the other two do not. The distal ends of the unbranched cilia are differentiated into tubular structures. The sensillum cone is comprised of an inner stem and a guard wall, neither of which is perforated. The outer cavity is enclosed by the 2nd, 3rd, and 4th enveloping cells, and distally in the antenna is closely apposed to the cuticular wall. This cavity contains lipid deposits. These structural features are discussed with regard to the mechanism of hygroreception in P. americana.