Four experiments were conducted to identify the possible loci of habituation of the nematode tap withdrawal response (TWR) by characterizing the effects of TWR habituation on other nonmechanosensory withdrawal behaviors that are mediated by overlapping sets of neurons. Experiments 1-2 established behavioral and anatomical relationships between spontaneous and tap-induced backward locomotion in the worm. Experiment 3 demonstrated that habituation of the TWR affected neither the magnitude nor frequency of spontaneous reversal activity. Experiment 4 extended this result to an evoked response: Habituation of the TWR had no effect on reversals evoked by a thermal stimulus. These studies, which show that the loci of change associated with habituation of the TWR are presynaptic to the interneurons and motor neurons that control locomotion, probably distributed among the mechanosensory neurons, illustrate that a complete understanding of plasticity requires a knowledge of both the anatomical and molecular substrates of change.