On the basis of four visits to the Grotte des Chauves-souris (12°57′S, 49°07′E), Parc National d'Ankarana, northern Madagascar, we examine patterns of sexual dimorphism and seasonal differences in body mass of Rousettus madagascariensis, an endemic Malagasy Pteropodidae. Two visits per season were made over the course of two years, dry season (September 2014 and 2015) and rainy season (January 2015 and 2016). Individuals of this species were trapped when exiting the cave after dusk or entering before dawn. Animals were aged, sexed, and then individually marked before being released. In total, 271 adult males and 289 adult females were captured. Although some overlap was found in measurement ranges, males showed significantly larger mean forearm length and body mass than females, including separate analyses of the four different capture periods and in the combined season dataset. For the September 2015 visit, for which a considerable number of adults of both sexes were captured at dusk and dawn, animals entering the cave before dawn were significantly heavier in body mass than those exiting after dusk. When body mass data were pooled within a season, significant differences were found between the rainy and dry seasons in males, as well as females, indicating fluctuating aspects. Further analyses of intra-season and inter-season showed differences in body mass, presumably associated with the availability of fruit resources. Further, the sex ratios of captured individuals were not equal during different capture sessions and inferences are presented on aspects of the breeding biology of this species.