Abnormal inter- and intraspecies aggression, perceived to be anxiety related, was identified in two male red-necked wallabies (Macropus rufogriseus) housed within a zoo. Aggressive episodes were directed at conspecifics, other exhibit animals, and, in one case, human caretakers. The clinical use of oral fluoxetine (0.5 mg/kg po bid) for a period of approximately 4 mo was effective in eliminating aggression towards humans and other animals in these two individuals. There was no evidence of recrudescence of aggression in either case following discontinuation of therapy for up to 3 yr posttreatment. Other than a period of mild transient sedation in one animal, side effects were not noted with fluoxetine treatment in these cases. Additional studies on the pharmacokinetics and side effects of fluoxetine treatment for anxiety behaviors are warranted in wallabies.