The ratio of the medulla to the cortex (M/C ratio) was determined for three species of Megachiroptera, and the ratio of the inner medulla to the cortex (IM/C ratio) was determined for 28 species of insect-eating Microchiroptera from diverse savanna and montane habitats within Malawi. Although all these species occur in the southern savanna biotic zone of Africa, each varies in its ability to exploit the African rainforest, arid and desert biotic zones. The kidneys of the Megachiroptera were very different to those of the insect-eating Microchiroptera, and these differences were related to the need of fruit-bats to excrete the surplus water associated with their diet. The urine concentrating capacity of each species of Microchiroptera was predicted from the IM/C ratio. Family characteristics were examined, and the urine concentrating capacities of the Malawian bats were compared with bats from elsewhere, and with some rodents. The insect- eating Microchiroptera exemplified a very diverse range of IM/C ratios. The IM/C ratio of each species was compared with its drinking behaviour in captivity, diet, foraging strategy, flight, aspect ratio, domicile, roosting behaviour and habitat. It was hypothesized that the ability of insect-eating Microchiroptera to conserve urinary water would be an advantage to species: (a) living in xeric habitats; (b) adapted for sustained high-speed flight; (c) roosting in hot dry domiciles; (d) roosting in solitude; and (e) living at high altitudes where seasonal low temperatures induce torpidity. These hypotheses were tested, and it was concluded that, while the ability to conserve urinary water may have been an advantage in most of these situations, it was not essential.