1. The white, bilaterally paired A1 interneurons of the cerebropleural ganglion of Pleurobranchaea californica fire rhythmic bursts of action potentials during escape swimming behavior. We studied the role of the A1s in swimming behavior and pattern generation in whole animal and isolated CNS preparations. 2. The escape swim is a cyclic sequence of dorsal and ventral flexions of the body. During the swim, A1 bursts precede and accompany the dorsal flexion phase of the cycle. Hyperpolarization of A1 to prevent spike activity interrupts swimming behavior in the whole animal and fictive swimming in the isolated CNS. Stimulated A1 activity was not observed to cause swimming in whole animals, and was only occasionally sufficient to trigger fictive swimming activity in the isolated CNS. 3. In quiescent whole animal preparations, stimulation of a single A1 normally causes a single dorsal flexion followed by body flexion to the side contralateral to the stimulated cell; characteristically, A1 spike activity stimulates feedback inhibition coinciding with the end of dorsal flexion and the onset of contralateral flexion. 4. A1 spike activity suppresses feeding behavior and causes proboscis retraction in whole animal preparations induced to feed. A1 activity also suppresses fictive feeding driven by stimulation of the critical phasic paracerebral neurons (PCps) of the motor network of feeding in the isolated CNS. Concomitantly, A1 spikes cause potent inhibition of the PCp interneurons. 5. The A1s are specifically excited by noxious mechanical and chemical stimuli, but are not affected by feeding stimuli or the occurrence of feeding behavior. 6. We conclude that the A1 neurons are elements of an escape swimming pattern generator, and that they are probably homologous to the similar C2 neurons of the nudibranch Tritonia diomedea. One of their functions outside of generating the swim pattern may be the suppression of feeding behavior in response to noxious stimulation. These observations provide a neural mechanism for the original observations of the dominance of escape swimming behavior over feeding.