BATTLE GROUND, Wash.—Sue Foster knew what she needed to do when her border collie, Taff, was expelled from puppy school for herding the black Labs into a corner.

She rented some sheep.

Then she bought another border collie and rented some grazing land. Then she bought some sheep of her own. And a third border collie. Now, like the old lady who swallowed the fly, Ms. Foster keeps a llama to chase off the coyotes that threaten the lambs that go to market to finance the sheep that entertain her dogs.

Once upon a time, Americans got dogs for their sheep. Now they get sheep for their dogs. "I never dreamed it would go this far," says Ms. Foster, 56 years old.

Border collies, first bred along the frontier between England and Scotland, are compulsive herders, with instincts so intense they sometimes search for livestock behind the television when sheep appear on screen, says Geri Byrne, owner of the Border Collie Training Center, in Tulelake, Calif. Left unoccupied, they'll dig up the garden, chew up the doggie bed or persecute the cat.