When Mayor Marion Barry named Carol B. Thompson yesterday as his new city administrator -- the top appointed job in the District government -- he held her hand aloft as if a victory had been won.

In fact, some of the mayor's aides say, the battle is just beginning.

Thompson, 36, who is highly respected and viewed as being fiercely independent in her current post as deputy mayor for economic development, is stepping into the new job just as Barry is grappling with pressing staff problems, sagging morale and administrative issues that will quickly test Thompson's abilities.

It's uncertain however, whether the mayor's free-wheeling political style will clash with Thompson's no-nonsense approach. "He got the {city administrator} that he needs, but not necessarily one that he wanted," one senior aide said.

Some aides recalled that Thompson, who served briefly as Barry's chief of staff in 1986, often challenged him and sometimes privately worried that he seemed to resist making hard decisions. Aides also said that Barry wavers between wanting hard-nosed decision-makers and more "rooters" -- Barry's term for aides who he believes are more supportive of him personally.

The mayor, speaking at a news conference attended by dozens of cheering city employes, tacitly acknowledged shortcomings in his administration that he said Thompson will try to help solve.

Barry vowed to fill by Feb. 1 eight other senior positions that are either vacant or held by acting directors, including deputy mayor for economic development, deputy mayor for finance -- which has been vacant nearly two years -- and director of finance and revenue.

Barry also announced yesterday that his part-time legal counsel, Herbert O. Reid Sr., would act as interim staff director until a permanent replacement can be found for Brenda Williams, who is scheduled to leave her post Dec. 31.

Barry has been criticized through much of the last year for low staff morale, but said the turnover in jobs "is not a sign of crisis, not a sign of abandoning ship, not a sign we're in difficulty or trouble. On this day, Dec. 18, we are stronger than ever before."

When one television reporter recalled that the mayor just a year ago introduced a new Cabinet team at the start of his third term in office, Barry said turnovers reflected his "dynamic government."

"Do you want it to be this dynamic?" the reporter asked.

"Not that dynamic," Barry said to laughter.

Thompson will succeed Thomas M. Downs, who has been city administrator for nearly five years. His salary was about $80,000 a year. Downs, who becomes president of the Triborough Bridge and Tunnel Authority in New York City next month, was praised by Barry yesterday as "a friend" and a "hard worker."

Some mayoral aides, however, expressed anger over a report that Barry privately has been critical of Downs and has blamed him for operational problems in the last year that have embarrassed the administration.

"Tom did a hell of a job, he doesn't deserve that," said one former high-ranking official familiar with city government.

Downs bowed out of the limelight during the news conference, which was held in the lobby of the Department of Employment Services building at 500 C St. NW.

Downs, who has worked in Barry's administration for seven years, said, "I feel like I've invested a good portion of my working life here. Even though it's been seven years, it seemed at times much longer than that."

Under the reorganization announced yesterday, the city administrator's post will be restored as the top appointed job in the city government. Downs, who also held the title of deputy mayor for operations, shared power with the other two deputy mayors' offices.

Thompson, who has said privately that she believes Downs was undercut by the shared authority, sought and won from Barry a commitment to restore the city administrator's powers.

Thompson, who has earned $70,000 and will be paid $77,000 in the new job, was greeted yesterday with sustained applause and cheers as she spoke about her commitment to the District, her voice occasionally wavering with emotion.

"I {want} to make a difference in my home town," said Thompson, who was born in Washington and graduated from its public schools. Thompson will become one of the country's highest ranking black women in public administration when she takes the day-to-day reins from Downs for the city's $2.6 billion government.

"With the role that she will be playing," an aide said, "she will be the bottom line on a lot of things. Hopefully, the mayor will listen."