The tap withdrawal reflex in Caenorhabditis elegans demonstrates various forms of nonassociative learning. A first step in determining the cellular mechanisms of this learning is to identify the neuronal circuitry that underlies this reflex. Studies by Chalfie et al. (1985) have defined the touch-circuit that mediates the response to a stimulus related to tap--a light touch. We used the touch circuit as a starting point in the identification of the tap withdrawal circuitry. Here we report the effects of lesions of identified neurons on the tap withdrawal reflex. Ablations of the sensory neurons and interneurons of the touch circuit produce effects on the tap withdrawal response that generally confirm and expand upon the roles of these cells in mechanosensory integration as proposed by Chalfie et al. (1985). However, no role for the LUA interneurons could be identified in the production of the tap withdrawal response. Furthermore, the effects of ablating some neurons outside the touch circuit suggest roles for two of these cells in the integration of the tap withdrawal response. Ablation of either the midline neuron DVA or the PVD neurons resulted in a decrease in both the frequency and magnitude of reversals that were elicited by tap. Additionally, the ablation of either cell decreased the magnitude of accelerations produced by animals in response to tap.