Aplysia californica has been extremely popular with neurophysiologists interested in understanding the neural basis of behavior. The major drawback to neuroethological work on this species has been the lack of information on its behavior. We present the first ethogram of this species and a model of the physiological organization of drives (motivational systems) in Aplysia. The ethogram of A. californica contains 45 action patterns, most of which involve relatively subtle movements or postures of the head. Some of these action patterns have been studied physiologically and we summarize that information. The action patterns are superimposable and an individual may perform several action paterns simultaneously. A. californica has five distinct "drives" or motivational systems: feeding, escape, reproduction as a male, reproduction as a female and spawning. The sequence of events involved in escape behavior in response to attack by Navanax (a natural predator) or other mechanical stimuli to the rear is described. Each "drive" activates a particular sequence of action patterns, and certain "drives" are superimposable. Aplysia can copulate as a female while copulating as a male, feeding, or laying eggs. On the other hand, escape is incompatible with female sexual behavior. The available physiological evidence suggests that neuropeptides may play an important role in the organization of drives and their interactions. We present a model which combines the available physiological and behavioral data with the classical ethological drive hypothesis to produce a testable model of the physiological organization of motivation in Aplysia.