I investigated spatial relationships and mating in a population of individually marked nine-banded armadillos, Dasypus novemcinctus, in a riparian habitat in south Texas. Adults had larger home ranges than younger individuals. Home ranges of adult females overlapped extensively with those of other adult females and breeding and nonbreeding males. Breeding males had almost exclusive home ranges from one another, but overlapped considerably with adult females and nonbreeding males. Observations of paired males and females indicated a well-defined breeding season, from June through November, with 83% of pairings occurring from June through August, confirming morphological data reported previously. The mating system was polygynous because most breeding males paired with more than one female during the breeding season, although most females paired with just one male. Polygyny and competition among males for mates was further suggested by sexual dimorphism in body mass and by the observation that just a few older males monopolized pairings.