Intra- and interspecific association of cattle and bonded or nonbonded sheep was observed under free-ranging conditions preceding, during and following the approach of a trained border collie dog. A bonded sheep is one which consistently stays in close proximity to cattle as a result of a continuous close association between the two species, which began when the sheep was a young lamb. The dog treatment provided insight into the response of livestock to an aggressive, threatening canine. Sheep bonded to cattle remained together as one interspecific group when threatened by the dog. Interspecific space decreased and the sheep positioned themselves among the cattle and away from the dog. Cattle aggression, i.e. kicking and charging the dog, was only observed when the dog approached the heifers. Nonbonded sheep and cattle reacted as two distinctly independent intraspecific groups. The nonbonded sheep reduced their intraspecific space and moved away from the cattle when threatened by the dog. The protection that bonded sheep receive from cattle appears to result from the close association with the cattle, which poses a threat to predators.