Migration and hibernation are survival strategies that require physiological preparation using fattening. Bats employ both strategies in times of resource shortages. However, because males and females vary seasonally in their reproductive physiological needs, they may employ different fattening patterns. Whilst fattening, migration and hibernation are common in temperate bats, little is known about subtropical migratory insectivores. This study investigated seasonal variation in body mass of the regionally migrating Natal long-fingered bat Miniopterus natalensis to determine if males and females show fattening in preparation for migration/hibernation. Seasonal change best explained the variation observed in overall body mass, whilst sex and forearm length explained the variation to a lesser extent. Body mass between males and females differed significantly by reproductive category among the four seasons. Forearm length was a significant predictor of the body mass of males. Scrotal males had a higher body mass in summer compared to autumn. This pattern of mass gain was not observed in non-scrotal males. The summer body mass of nonpregnant and post-lactating females was not significantly higher than the autumn body mass of nonpregnant females, which did not support the hypothesis that females would exhibit fattening during summer before migration. Results suggest that males and females employ different mass-gain strategies related to reproductive investment rather than fattening preparation for migration or hibernation.