The locomotor activity cycle of the American eel (Anguilla rostrata) was studied in a light-proof shelter. Measurements of activity were registered by a mechanical assembly triggered by the eel’s movements. The results demonstrate that the eel posses a locomotor cycle with greater activity in dark than in light and activity peaks in response to light changes at dawn and dusk. The latter is significantly higher than any of the peaks during the night or the one at dawn. The one at dawn is frequently not any higher than some peaks of activity occurring throughout the night. The presence of peaks at dawn and dusk appears to be a more generalized condition than the period of maximum activity. Both frequency and phase of the eel’s cycle were shown to vary directly with the light conditions, and the cycle was totally obliterated under constant conditions of either light or dark. It is concluded that this locomotor activity cycle is essentially exogenous, under the control of the environmental light cycle, with the preference for dark activity being the only endogenous component.