The latest discovery came at a time when US officials were trying to convince Japan to reopen its market to US beef after a shipment of veal violated meat export rules by containing spinal material. Japan suspended beef trade on Jan. 20.

South Korea said earlier on Monday it could cancel a planned opening of its market in April to US beef if a new US case was found. Seoul has banned US beef since the first US case in December 2003.

Agriculture Department Chief Veterinarian John Clifford announced the positive test results during a telephone news conference. He said the new case was confirmed through the Western Blot test, which was added to USDA's battery of tests last year.

But Clifford insisted that the discovery of a new case of mad cow disease in the United States should not scare other nations away from American beef.

"We have sufficient safegaurds" to assure US beef is safe to eat, Clifford said. He said USDA hoped "this would not affect" trade.

USDA had few details about the cow's home farm or who owned the animal before she arrived at the Alabama farm less than a year ago. The nonambulatory cow was euthanized by a veterinarian who saved brain samples for testing.

The United States has struggled to restore beef exports to overseas markets that totaled $3.8 billion annually before mad cow was discovered. Beef exports in 2006 are estimated at 905 million pounds, down 64 percent from 2003.

The cow appeared old enough to have been born before the 1997 ban on using cattle parts in cattle feed - one of the two major US safeguards against spread of the disease. The other is a requirement for meatpackers to remove from carcasses of older animals the brains, spinal cords and other nervous tissues most at risk of carrying the infective agent.

Before the positive test result was reported, Agriculture Secretary Mike Johanns, traveling in Europe, said the United States had a wide-ranging set of safeguards against mad cow.

"I do not think it will have a negative impact on trade with other countries," Johanns told reporters during a visit to Poland.

After the United States discovered its first case of mad cow disease in December, 2003, USDA launched an enhanced testing program of cattle believed to be at the highest risk for the disease. The latest suspect animal was found as part of the program, which so far has tested more than 650,000 cattle since June 2004.