Effects of rearing conditions were examined in the development of agonistic behaviors in the male fighting fish. In group-I (highly social), fish were communally reared. In group-II (highly social and isolate), fish were individually housed and exposed to the group-I fish through transparent walls until the sexual maturity (from 6 to 12 weeks post-hatch). In group-III (social and isolate), individually housed fish were similarly exposed to other fish within the group. In group-IV (non-social and isolate), individually housed fish were further visually isolated. Agonisitc behaviors were compared among males of the groups-II, -III, and -IV in their fights against the group-I male. The group-IV males showed significantly higher rate of wins than the groups-II and -III males, without differences in the incidence of agonistic behaviors (butt-or-bite, chase, and gill-cover erect) before the termination of the mutual fights. Increased incidence of agonistic behaviors was found after the termination (particularly in the unilateral chase), suggesting that the group-IV males continued to fight even after the opponent male displayed a submission. The aggression was also enhanced in the group-II, when they were thereafter reared in a social isolation after the sexual maturation; a critical period was thus not found. The enhanced aggression was not reversed in the group-IV, when they were thereafter exposed to social stimuli; shift to the continued fights was irreversible. Possible fitness gain of the enhanced aggression was discussed in terms of the adjustability to altered biological resources.