Where's the Beef?

What is "beef bowl" without the beef? That sounds like a Zen riddle, but it's actually the nightmare playing out at Yoshinoya D&C Co., Japan's leading purveyor of gyudon , a fast-food staple of rice topped with thin slices of stewed beef that's tasty, filling and, at just $2.60 per serving, fantastically popular with students and salarymen.

Fast-food giants such as McDonald's procure beef from all over, but Yoshinoya imports almost 100% of its meat from cheap U.S. producers. When Japan banned imports of American beef in late December because of mad-cow disease, Yoshinoya chief executive Shuji Abe called...