1. Mechanosensory hairs on the surface of the crayfish telson are dually innervated, one sensory cell responding to headward, the other to tailward deflection of the hair. The average conduction velocity of headward elements was 0.8 m/s (variance 0.08) and of tailward elements 1.2 m/s (variance 0.19). In a frequency range from 0.05 to 200 Hz, thresholds were lowest near 20 Hz: 0.08 mum (pp) for headward-sensitive and 0.1 mum (pp) for tailward-sensitive cells. 2. The receptors are displacement sensitive since thresholds are of the same order of magnitude over the frequency range 1-70 Hz when the hair is moved by a vibrating wire loop. With natural stimuli (surface waves), the velocity component of the particle movement (and consequently force) becomes influential. The coding of a broad range of stimulus intensities is aided by variations in mechanical properties of the hair. 3. Marked directionality (better than 4:1), in addition to the dual innervation, enhances vector detection. At least part of this characteristic stems from the hingelike articulation of the hair on the body surface: the hair can be moved easily 40 degrees tailward and 20 degrees headward, but must be forced in the orthogonal direction. Morphological studies indicate the presence of a double pivoted hinge, with rigid guides for movement of the hair shaft. Preliminary results of electron microscope examination show a clearly polarized arrangement of densely packed microtubules in the two dendrites; they appear interconnected in groups of two and three along a line parallel to the sensitivity plane of the receptor. 4. The 50-fold threshold difference between the results of behavioral experiments in lobsters (24) and the data for the individual receptors reported here may be due to improvement in signal-to-noise ratio by central nervous averaging of the input from an estimated 2 X 10(3) receptors (Procambarus), and/or to the kind of threshold criteria applied to individual receptor thresholds. As would be expected (35), the sensory cells of each directional class synapse with separate interneurons: in this way, the organism might employ differential microphones to reduce background noise. 5. The receptors are analogous to those of the lateral-line system in lower vertebrates in having receptors with sensitivities polarized by 180 degrees. These similarities suggest that in both cases monitoring of near field water displacements has proved in essential way of orienting in opaque waters.