The fine structure of the prothoracic and mesothoracic femoral chordotonal organs of a grasshopper, Romalea microptera, is described. A single chordotonal organ may contain more than 300 sensory units. Each unit includes the cell bodies and dendrites of two neurons, together with sheath cells of several kinds. The cytoplasm of the sheath cells is packed with microtubules and filaments. These cells surround the cell bodies of the neurons and anchor them to the inner surface of the femoral wall. The dendrites from the two neurons are enclosed by a scolopale cell. At the distal end of this cell they traverse the extra-cellular space within the scolopale and their tips are fitted into cavities in the scolopale cap. The ciliary region of each dendrite is dilated for about one fourth of its total length. The cap is embedded in an elongate cap cell which is attached, in turn, to a ligament cell. The ligament cells are, themselves, attached to an apodeme that extends in from the tibia. When the tibia is flexed, the chordotonal organ is stretched and when it is extended, the organ is relaxed. It is postulated that the mass of the dilated region affects the character of the vibration that is induced when the dendrite is stretch or relaxed.