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Sperm competition is a widespread phenomenon influencing the evolution of male anatomy, physiology and behaviour. Bats are an ideal group for studying sperm competition. Females store fertile sperm for up to 200 days and the size of social groups varies from single animals to groups of hundreds of thousands. This study examines the relationship between social group size and investment in spermatogenesis across 31 species of microchiropteran bat using new and published data on testis mass and sperm length. In addition to male competition, I examined the effects of female reproductive biology on characteristics of spermatogenesis. Comparative studies indicate that relative testis mass is positively related to sperm competition risk in a wide range of taxa. Social group size may also influence the level of sperm competition, and one of the costs of living in groups may be decreased confidence of paternity. I used comparative analysis of independent contrast (CAIC) to control for phylogeny. Using two possible phylogenies and two measures of social group size, I found a significant positive relationship between social group size and testis mass. There was no relationship between testis mass and the dimension of the female reproductive tract or oestrus duration. Sperm length was not significantly related to body mass or group size, nor was it related to oestrus duration.