1. The tail withdrawal reflex is mediated by a monosynaptic circuit composed of tail sensory and motor neurons, but there appear to be additional neuronal elements that also contribute to the reflex. A newly identified interneuron, called LP117, was located in the pleural ganglion. This neuron formed a parallel excitatory pathway between sensory and motor neurons. The distinguishing feature of LP117 was its ability to elicit a long-lasting (5-100 s) excitatory postsynaptic potential (EPSP) in the motor neuron. 2. Intracellular labeling of LP117 revealed axons projecting to the cerebral and abdominal as well as the pedal ganglia. Simultaneous intracellular recordings confirmed the widely divergent output of LP117 to tentacle motor neurons in the cerebral ganglion, as well as to gill, siphon, and ink motor neurons in the abdominal ganglion. 3. Also receiving input were abdominal neurons L29, which excites LFs motor neurons and facilitates LE sensory neurons, and L25, which is part of the pattern-generating network underlying respiratory pumping. Thus LP117 appears to be a neural element important for the conduction of information about tail stimulation to ganglia that are not innervated by tail sensory neurons themselves. Moreover, the divergent outputs suggest that LP117 is an element of a neural circuit underlying defensive arousal. 4. LP117 produced slow EPSPs in several motor neurons. The long time course of the EPSP could prolong the burst in the motor neuron produced by LP117 itself as well as increase the effectiveness of coincident synaptic input. This suggests that an important function of this interneuron is to extend the duration of the response to tail stimulation in the motor neuron. This could account for the relatively long time course of the motor neuron response to tail stimulation compared with that of the sensory neuron. 5. Sensitization is a form of nonassociative learning that produces changes in the amplitude and duration of reflex responses. It seems unlikely that all of these changes can be attributed to enhanced amplitude of the sensory-motor synapse, however. Therefore LP117 may itself be a site of plasticity for reflexes elicited by tail stimulation.