Scent-marking behaviour and inter-pack aggression were studied in wild Ethiopian wolf, Canis simensis, packs. Raised-leg urinations, followed by ground scratching, were the most frequently deposited scent-mark. Scent-marking rates were highest along or near territory boundaries, where distances between scent-marking sites were reduced and the proportion of multiple marks was increased, relative to other areas. Marking rates increased with wolf numbers during patrols but not during other activities. Although all adult members of a pack contributed to scent-marking, the dominant pair marked most frequently. Subadult males scent-marked occasionally but subadult females never did. Wolves vigorously over-marked neighbours' scent-marks. Most direct encounters between neighbouring wolves at territory borders were aggressive and involved repeated chases, and the larger group was most likely to win. Resident wolves were more tolerant of opposite-sex than same-sex neighbours. Resident wolves therefore signalled pack composition and status at home range borders by olfactory and auditory cues and by aggressive contests. Such signalling may reduce the occurrence of potentially costly inter-pack aggressive encounters at territory borders and provide information on reproductive status.