The somatic setae of Chromadorina bioculata and Enoplus communis have been examined with the electron microscope. Their structure is in agreement with their function as mechanoreceptors. The cuticular elements of the setae are weakened at their base and they appear to pivot in a cuticular depression. The sensory element is a modified cilium, which takes the form of a typical (9 + n) microtubular organisation at the base of the setae. These cilia connect with nerves which proceed into the lateral cords. The wider possibility of mechanical deformation of the cuticle being transmitted to the setae is discussed.