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Notes: Summary 1. Electrophysiological recordings from individual auditory sensilla in the tettigoniid (Caedicia simplex) ear (Fig. 1) show that spike potentials of two distinct amplitudes occur in the sensory neuron (Fig. 2). Generally, the smaller of the two classes of spike is observed as an inflection on the rising phase of the larger spike. In cases where the larger spike fails or is blocked, the smaller spike becomes distinct. In the attachment cell of the sensillum, negative-going monophasic, or biphasic spikes are recorded (Fig. 3). In biphasic spikes, the positive phase is of smaller amplitude and always follows the negative phase. 2. Simultaneous recordings from the sensory neuron and from its associated attachment cell, using two electrodes, show that the negative-going spike recorded from the attachment cell corresponds with the smaller spike recorded from the neuron (Fig. 4). The occurrence of a large spike in the neuron causes the positive-going potential in the attachment cell, immediately following the negative spike. 3. Injection of negative current into the attachment cell via a recording electrode elicits spikes in the sensory neuron. If large spikes in the neuron fail or are blocked by hyperpolarization of that cell, the injection of negative current into the attachment cell elicits small spikes in the neuron (Fig. 4). 4. Electrical stimulation of the tympanal nerve induces retrograde spikes in the soma of the sensory neuron. Such spikes show no inflection in the rising phase, indicating the absence of the small spike as a precursor to the retrograde spike (Fig. 6). Recordings from the attachment cell, when retrograde spikes are induced, show only a positive-going potential correlated with each retrograde spike. The positive deflections recorded in the attachment cell, resulting from retrograde spikes, generally are of greater amplitude than those specifically associated with large, orthograde spikes occurring in the sensory neuron. 5. These results confirm previous suggestions that, in insect auditory, chordotonal sensilla, spikes of relatively small amplitude occur at the apex of the sensory dendrite and subsequently, trigger spikes of conventional amplitude at a site more proximal in the dendrite. The occurrence of small spikes in the neuron implies novel equilibrium potentials at the apical dendritic membrane, which may result from the scolopale lumen functioning as a receptor lymph cavity.