The behavior of a nocturnal malagasy Prosimian (Phaner furcifer) in its natural environment has been studied in relation to its visual abilities and to the light cycle of the biotope. By photometric measurements (autonomous photomultiplier used with interferential filters) we attempted to connect the activity rhythms of the animals with the light level in the actual biotope (quantitative and qualitative approach). 10 Emergence from the nest occurs when the level of light becomes sufficiently low (about 1 lux). Return to the nest is observed as soon as the light level increases. The times of these behavioral activities therefore vary with the season. 20 Histological examination of the retina reveals only the presence of rods. The absolute sensitivity threshold (scotopic) measured by a technique of operant conditioning is comparable to that of man. 30 The level of light observed in nature is well above the visual threshold. The precision with which emergence from the nest occurs in relation to the ambiant light is surprising considering the absence of cones in the retina.