At exactly 10 a.m. yesterday, with a telltale clacking of heels from a trio of spit-and-polished soldiers, the honor guard changed at the Tomb of the Unknowns in Arlington National Cemetery. This is a time-honored ceremony, little altered in the 48 years that the Army's elite 3rd U.S. Infantry, the Old Guard, has been keeping its round-the-clock vigil at the white marble sarcophagus.

Except that yesterday, the Old Guard became the new guard.

Few in the crowd of camera-toting tourists realized that they were witnessing history in the making while they huddled in the brisk air and watched the first female soldier take her place at the hallowed site.

"This is a great day for the Army," proclaimed Capt. Michael Eddings, commander of the Old Guard, as Heather Lynn Johnsen, a 23-year-old sergeant from Roseville, Calif., became the 389th soldier in 38 years to receive the silver tomb guard badge, one of the military's most prestigious emblems.

Johnsen stood ramrod straight in her dress blues, her long brown hair neatly braided and pinned up in back, as a small media mob clustered around her shortly before her first official walk at the tomb. Her face betrayed no emotion, not even a smile when her commanding officer, checking the buttons on her coat, said, "That's a tight fit there."

Tomb sentinels are known for their steely demeanor on duty. Cracking even a slight smile can be grounds for dismissal, and Johnsen was not about to risk all that she's worked for in the last nine months.

In sentences as measured and precise as the 90-steps-per-minute cadence that sentinels pace off at the tomb, Johnsen said after an awards ceremony that she felt "overwhelmed, relieved, extremely happy and extremely honored." At another point, she offered, "I can't think of anything else I'd rather do for my country than guard the Unknowns."

Although women were admitted to the Old Guard's Fife and Drum Corps in 1982, it wasn't until two years ago that the secretary of defense directed the Army to allow them to apply for the guard's three elite units, including the tomb sentinels.

Beyond standing watch over the tomb, which honors the nearly 90,000 unknown American servicemen and women who have given their lives for their country from World War I through Vietnam, the guard's 1,273 members also perform funerals at Arlington, take part in ceremonial functions and greet foreign dignitaries.

Formed in 1784, the Old Guard is the Army's oldest active unit, and the decision to allow women stirred controversy.

Johnsen said she was treated like any other sentinel in training and received no special breaks. She met every requirement, including the 5-foot-10 minimum height. "I wouldn't be here today if I hadn't proved myself," she said, and many of her male colleagues shared her assessment.

Some are clearly unhappy, however. "We opened the henhouse to the wolf," said Matthew McKaig, applying a malapropian twist to the fraternity of sentinels. McKaig, who mustered out of the Army yesterday at the same hour Johnsen stepped into the limelight, set an Old Guard record last month with his 1,121st vigil at the tomb.

It was the break with tradition that rankled him and other male sentinels. "There's a certain image," McKaig said. "It's like the guards at Buckingham Palace. You don't expect to see a woman there."

Nevertheless, the 21-year-old Marylander allowed that Johnsen had earned her place. "She was good," he said.

So good, in fact, that she received a perfect score of 100 Wednesday on her final exam, a grueling test of Arlington lore that requires memorizing the history of the cemetery and the exact location of nearly 200 burial plots among the 240,000 people interred there.

Most tomb guard applicants never get that far, with 70 percent dropping out during the first two weeks.

Although she made 179 tomb walks during her training, including several during off-peak hours, yesterday's was for real. "It finalizes everything," Johnsen said.

Several in the crowd said they felt honored to have shared in the milestone moment.

"I think it's awesome to have a woman there," said Matt French, 17, visiting Washington with his high school band from Buhler, Kan.

Jessica Wallace, 10, chimed in: "It shows women are tough, too, and can guard just as good as men can." Jessica and her classmates from Danville, Ala., had the good fortune to make Arlington their first stop on arriving in town yesterday.

"We just hit it accidentally, and I'm so glad," said her principal, Gary Walker.

Joe McCarville, West Point Class of 1969, wore the academy's motto stitched into his cap.

He noted that times have changed, and the Army has, too.

"I think it's important for women to have these opportunities and for kids in school to see this, the role model thing," he said. Johnsen seems comfortable filling that role, saying she hopes -- and expects -- to be joined by other female tomb guards. "I would hate to think of myself as the one and only." CAPTION: Sgt. David Banks, left, inspects the rifle of Sgt. Heather Lynn Johnsen during her first changing of the guard ceremony at the Tomb of the Unknowns in Arlington National Cemetery. Johnsen, 23, received a perfect score on the exam to join the elite ranks. CAPTION: Spectators strain to glimpse Sgt. Heather Lynn Johnsen as she prepares to leave a hut during her shift standing guard at the Tomb of the Unknowns. CAPTION: The Guard Changes Tourists watch as Heather Lynn Johnsen, a 23-year-old sergeant from Roseville, Calif., becomes the first woman to guard the Tomb of the Unknowns in Arlington National Cemetery. "This is a great day for the Army," said Capt. Michael Eddings, her commander. (Photo ran on page A01)