Potential determinants of aggression in nine-banded armadillos, Dasypus novemcinctus , were identified by comparing the sex, age, and reproductive status of initiators and recipients of aggression in a marked population in southern Texas. Males and females were equally likely to initiate and receive aggression. Adults initiated most aggression, directing it towards younger animals. Adult males directed aggression primarily at 1- and 2-year-old males and did so most frequently during the breeding season (June–November). Most adult males exhibiting aggression were previously or subsequently seen paired with females, suggesting that male aggression may function to maintain exclusive access to receptive females. Adult females directed aggression primarily at juveniles of both sexes and at other adult females, mostly during a 4-month period corresponding to late pregnancy and lactation (February– May). These findings suggest female aggression may function in defending current litters and in promoting the dispersal of last year's young. Examination of other potential influences on agonistic behavior showed that heavier individuals were more likely to win encounters and that escalated contests (i.e., fights) were more likely between evenly matched opponents.