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Abstract We analysed post-natal growth data for body mass from 33 species of free-ranging and captive bats, using the logistic growth equation. When these data were examined by means of linear regression and covariance analysis, we found that growth rates decreased linearly with increasing asymptotic body mass. When we removed the effect of body mass, growth rates showed no significant differences with respect to diet (insect or fruit), taxonomic affiliation (Megachiroptera or Microchiroptera), growth condition (captive or free-ranging), or basal metabolic rate. Climate (tropical or temperate) was the only variable that had a significant effect on post-natal growth rates, with temperate bats growing faster than tropical species. This climatic effect was also evident when insectivorous bats were examined separately. While post-natal growth rates may provide a valuable index of maternal investment, milk energy output of females during lactation should provide the most direct link between the environment and growth of pups. Milk composition and milk-energy output as indices of maternal investment have been investigated in only a few species of bats. From the limited dat available it appears that the milk of insectivorous species contains a higher percentage of dry matter, fat and protein than does that of frugivorous species.