1. A pair of identifiable symmetrically placed neurons (CeW) in the cerebral ganglion of the nudibranch mollusk, Tritonia diomedea, exerts strong direct control over the generation of the rhythmic escape-swimming motor pattern. 2. In intact animal preparations, the CeW neurons produced action potential bursts immediately preceding each flexion cycle (Fig. 1). 3. Direct brief stimulation of a single CeW neuron elicited one or more complete flexion cycles. Responses of more than one flexion cycle corresponded to recurrent bursting activity in the stimulated CeW neuron. In addition, the response included aspects of the preparation to swim and its proper termination (Figs. 2, 3). 4. Inhibition of both CeW neurons to prevent the firing of action potentials prior to stimulation of the swimming behavior prevented the generation of the motor pattern directing swimming and termination but not preparation (Fig. 4). 5. The great majority of interactions between CeW's and other central neurons appeared polysynaptic, including a long-lasting inhibition exerted by the CeW's over many neurons not obviously involved in the swimming pattern (Fig. 6). 6. A depolarizing excitation plateau was seen in CeW neurons during their bursting activity which has the same time course as that seen in motoneurons. In one instance, direct stimulation of a dorsal flexor motoneuron reliably elicited epsp volleys in a contralateral CeW with a one-second latency. No ventral flexor motoneurons elicited a similar regenerative response. 7. The CeW neurons function at a dominant controlling level in what may be an interneuronal pattern generator. They may act to inhibit certain competing behaviors as well.