The locomotory activity of adult cave fishes Astyanax jordani was recorded in isolation in the following light and temperature conditions: constant conditions (100 Lx - 20°C), in a light cycle (LD:11/11 -10 Lx -100 Lx) and in a temperature cycle (11/11; 17/20, 19/22, 20/23, 27/30 °C). All longitudinal time series extended for a minimum of 30 days. Results show: (1) That no circadian regulation appears in constant conditions; (2) that passive entrainment occurs in LD (Amplitude: 90 Lx) and in periodic temperature conditions (Amplitude: 3°C). The entrainment effect damps out and varies individually; (3) that the mean activity increases with temperature; (4) The adjustment of activity to periodic signals is individually stable. These results suggest that A. Jordani is devoid of any endogenous oscillator of the circadian type. The observed thermal adaptation could have the following functions: (1) To increase the level of activity in function of the thermal level under the form of passive entrainment; (2) To enhance the exploratory behaviour of the fish in search of a thermal preference allowing the animal to keep inside a well defined zone of the subterranean biotope in relation to small local temperature changes.