This study examines seasonal behavioral activity in a cave day roost of an endemic Malagasy fruit bat, Rousettus madagascariensis (Chiroptera: Pteropodidae). Activities of both sexes (male and female) classified by age (neonate, sub-adult, and adult) were filmed in the cave using an infrared camcorder during the wet and dry seasons. Analyses of the videos were used to calculate the proportion of time spent conducting predefined behavior types during different 10 min periods. Measurements taken from captured individuals were employed to calculate the body condition index (BCI, ratio of body mass to forearm length) for the different age and sex classes. The results indicate that cave-roosting behavior changed as according to season. During the dry season, roosting individuals spent more time in ‘rest’and ‘consume ectoparasites’ behaviors and limited ‘move’ (flight and crawl) and ‘groom’ activities. Further, the BCI was lower during this period, as compared to the wet season, and this shift is presumably related to food availability or quality. During the wet season, ‘groom’ behavior frequency was higher and individuals formed a more dispersed roosting group configuration. The threshold for the shift to ‘tight’ group configuration and less grooming activity is correlated with climatological variables, specifically when the average temperature outside the cave was ≤ 25.3°C, the mean temperature inside the cave was ≤ 24.0°C, and the average relative humidity was ≤ 68.2%. In contrast, the presence of solitary roosting individuals and ‘loose’ group configuration were more common when the average temperature outside the cave was ≥ 27.1°C and the average temperature and relative humidity inside the cave was ≥ 25.0°C and ≥ 93.4%, respectively. Future studies should examine aspects of thermoregulation in cave-roosting pteropodids living in tropical areas, combined with measures of food availability and quality, and the relationship of these variables to reproduction.