This thesis concerns genome evolution in mammals, using the order Chiroptera to test hypotheses concerning the influence of biological traits on genome size. In 164 species of bats, genome size varied from 1.63 pg in ' Lophostoma carrikeri' to 3.17 pg in 'Rhinopoma hardwickii', averaging 2.35 pg ± 0.02SE. 43 estimates represented species from the previously unstudied family Pteropodidae ("megabats"). Consistent with previous hypotheses on genome evolution in bats (and flying vertebrates), bats have constrained genomes relative to other mammals, implying a link between genome size and the origins of flight. Surprisingly, megabats have even more highly constrained genomes than microbats. However, unlike patterns observed in avian species, bats did not show any relationships between genome size and metabolism, or flight-related characteristics. Rather, body size (or some other trait relating to it) seems to be a more relevant biological factor explaining the current extent of genome size variation in bats.