In order to better understand the symbiotic relationship of the oxpecker-mammal association and the role that oxpeckers play in controlling their hosts' tick burdens, interactions between yellow-billed oxpeckers (Buphagus africanus) and impala (Aepyceros melampus) were investigated at Matobo National Park, Zimbabwe during the wet and dry seasons. Oxpeckers devoted 30-35% of attendance time to foraging upon impala hosts. The ears were preferred for foraging above all other body regions, and foraging sessions directed to the ears were longer than sessions on other areas, apparently due to high tick infestation on host ears. Two-thirds of adult ticks (mostly the blue tick Boophilus decoloratus) collected from impala females were from the ears, and heavy infestations of immature ticks on the ears were common. The majority of oxpecker foraging (71-74%) was directed to the ear, head, and neck area where impala are unable to self-oral groom. Most adult ticks (75-77%) were found on the ears, head, and neck of sampled impala, indicating that oxpeckers foraged so as to maximize adult tick intake. Adult tick abundance in the vegetation, and presumably on impala, was much greater in the wet season than in the dry season. Oxpeckers spent significantly less time foraging upon impala in the wet season compared with the dry season, reflecting the presumed greater abundance of adult ticks on hosts during this time. Impala hosts tolerated oxpeckers 86% of the time, and 42% of oxpecker-tolerant impala accommodated foraging activity by lowering an ear, inclining the head, or standing still. An interaction was apparent between the tick-removal strategies of oxpeckers and their impala hosts in that impala reduced their grooming rate when oxpeckers foraged upon them to 11-36% of their grooming rate in the absence of oxpeckers, thereby reducing the cost of tick control.