1. A simple activity wheel for automatic and continuous recording of the locomotor activity of aquatic or marine animals is described (Figs. 1 and 2). 2. Aplysia californica is diurnally active, but shows a tendency to remain active for a few hours after the light-dark transition. The Circadian Rule states that for light-active animals spontaneous frequency, the activity:rest ratio, and total activity increase with increasing intensity of continuous illumination.Aplysia, obeys the Rule in two parameters tested; period and total activity. 3. Power spectrum and periodogram analysis of the activity show that there are several periodic components of activity (Figs. 5 and 9) in LL. 4. LD entrainment pulls these components into a major peak at 24 hours (Fig. 10). 5. Three circadian rhythms, one in the central nervous system, one in the eye, and the locomotor rhythm are known forAplysia. This animal could prove exceptionally useful in studies of internal temporal organization of circadian rhythms.