Abstract Few studies have examined the role of steroid hormones in mediating tactic-associated differences in male agonistic behaviour. Long-term (24 h) steroid hormone responses to winning staged male–male encounters in different morphs of free-living male tree lizards, Urosaurus ornatus, were studied. Males belong to either an aggressive, territorial (type) morph or to a less aggressive, non-territorial morph. On the day after winning a staged male–male encounter, males of the less aggressive morph showed elevated plasma levels of the stress hormone corticosterone and depressed levels of the sex steroid testosterone compared with controls of the same morph not subjected to an encounter. In contrast, males of the more aggressive, territorial morph that engaged in an encounter had levels of both hormones similar to controls. This morph difference in hormonal response is the first endocrine difference between adult males of the two tree lizard morphs detected to date; previous field and laboratory studies showed no morph difference in resting hormone levels or in hormonal responses 30 min following male–male encounters. The consequences of these morph differences in hormone responses to male–male interactions are unknown, but the hormonal changes in the less aggressive morph may (1) influence spontaneous agonistic behaviour, (2) influence agonistic behaviour in subsequent male–male interactions or (3) reflect morph differences in physiological capabilities related to energy expenditure during male–male interactions.