Male Grey hairstreak butterflies employed two different tactics, centred on palo verde trees growing on hilltops in central Arizona. During much of the spring flight season of about four months single males repelled all others from a perch tree. Interactions between the territorial resident in a tree and visiting intruder males were frequent but usually brief. One male retained possession of his perch for two weeks, although changes in ownership were generally much more frequent. But territorial behaviour was abandoned during periods when the number of intruders increased sharply, at which time perched males switched to non-aggressive patrolling about the preferred palo verdes. Females occasionally visited the palo verdes and interacted with males but no matings were observed. The mating system of the Grey hairstreak, like that of other similar landmark-defending insects, appears to have evolved in response to a diffusely distributed population of receptive females.