The active neuronal populations in the Aplysia abdominal ganglion during spontaneous and evoked behaviors were compared with the use of multineuronal optical measurements.In some preparations, more than 90 percent of the neurons activated during the reflex withdrawal of the gill also were activated during respiratory pumping and during small spontaneous gill contractions. Although the same neurons made action potentials in all three behaviors, the activity patterns were different. There was a substantial interaction between the neural substrates underlying evoked and spontaneous behaviors when they were made to occur together. If a gill withdrawal reflex was elicited a few seconds after a respiratory pumping episode, the evoked neuronal activity in most neurons was clearly altered. These results suggest that a distributed organization involving a large number of neurons may be responsible for generating the two behaviors. Different behaviors appear to be generated by altered activities of a single, large distributed network rather than by small dedicated circuits.