We observed mating by Brazilian free-tailed bats (Tadarida brasiliensis) in central Texas between 21 March and 5 April 1998. We documented copulations in large and small day roosts and in temporary night roosts. Focal animal sampling at a highway bridge revealed an aggressive and a passive male copulation strategy that may function as adaptations to different roost conditions. During aggressive copulation, the male separates a female from a roost cluster and restricts her movements during mating while he emits characteristic calls. During passive copulation, the male moves very slowly onto a female that roosts in a dense cluster. Passive copulations occur without resistance from the female and without male vocalizations. Both males and females mate with multiple partners, suggesting that mating is promiscuous. The mating system in a large highway bridge colony is characterized as mating aggregations or swarming because mating occurs in large, temporally unstable multimale and multifemale mating groups, with no apparent male territories or defense of females.