The domestic ferret is an increasingly popular pet. They are usually tame, playful, curious, and quite docile. Fights between ferrets can, however, result in serious injury. This study examined fighting between pairs of ferrets placed together in a pen. While cage mates did not engage in serious fights, almost 60% of newly introduced ferrets engaged in fights that included shaking, dragging, screaming, fleeing, urinating, or defecating. An attempt was made to identify the variables that affect the likelihood of aggression between ferrets and to design a method of introducing ferrets that would reduce fighting. Unfamiliarity was the strongest predictor of fighting behavior. The time of year (spring vs. winter) did not affect fighting behavior. Males and intact (i.e., not neutered or descented) animals were not always more aggressive than females or neutered animals. Intact male/intact male pairings were significantly more likely to result in a fight than intact male/neutered female pairings. Neutered female/neutered female pairings were significantly more likely to result in a fight than intact male/neutered female pairings. For pairs that did fight, a two-week familiarization period, during which pairs of ferrets lived side by side separated by wire mesh and rotated between sides every 24 hours, did not result in less fighting than in the control group. In conclusion, ferrets that are already cage mates are the least likely to fight. When introducing neutered pet ferrets, male/male pairings and male/female pairing are expected to fight less than female/female pairings. Finally, all introductions should be monitored.