PARIS, Sept. 28 — When President Bush told the United Nations General Assembly on Sept. 12 that the United States would rejoin Unesco after an 18-year absence, the announcement went almost unnoticed. In a speech dominated by his call for international action against Iraq, Mr. Bush dedicated just 37 words to the end of the American boycott of the United Nations Education, Scientific and Cultural Organization.

But this announcement had been long awaited by Koichiro Matsuura, 65, the soft-spoken Japanese diplomat who has been Unesco's director general since November 1999. Having shaken up and slimmed down the organization, he had finally convinced Washington that Unesco no longer resembled the bloated, corrupt and left-leaning organization that the United States abandoned in disgust in 1984.

Now, he believes, with the United States soon to return to the organization's headquarters here, Unesco can at last turn its energies away from internal overhaul toward tackling critical educational, environmental and cultural problems.

"It will make an enormous difference," Mr. Matsuura said. "Dealing with global issues, you need the participation of the global power. Until now, as observers, the United States was not part of decision-making or follow-up. For global action, we need the United States."

How American participation will change Unesco, though, is still a matter of speculation. In his speech on Sept. 12, Mr. Bush looked forward.

"As a symbol of our commitment to human dignity, the United States will return to Unesco," he said. "This organization has been reformed, and America will participate fully in its mission to advance human rights and tolerance and learning."

Yet while his words echoed those drafted by American officials in 1945 for the preamble of Unesco's constitution, they also illustrated both the vastness and the vagueness of the mandate of an organization that, by United Nations standards, is small and poorly financed.

"Unesco is not a financing institution," Mr. Matsuura said. "It is more of a think tank. It should be measured by its programs and ideas."

Thus, because people represent Unesco's main capital, 60 percent of its annual $272 million budget goes to administration, and it is also vulnerable to bad management at the top. For the United States, the crisis peaked between 1975 and 1987 when, as director general, Amadou Mahtar M'Bow of Senegal politicized Unesco, notably by promoting a "new world information order" — something Western governments viewed as a move to crush press freedom.

His successor, Federico Mayor of Spain, abandoned this program and, with the end of the cold war, distanced the organization from its previous anti-Western grandstanding. He tried to woo the United States back to the fold and was encouraged when Britain, which also walked out in 1984, resumed its seat here in 1997. But by the time Mr. Mayor retired in 1999, he was being criticized by Western governments for mismanagement.

"It was in very bad shape," Mr. Matsuura said of the organization he inherited. "In terms of management, it was much worse that I anticipated. My predecessor had around 30 special advisers, 5 of them very powerful. They were described as consultants, but they were more powerful than the assistant directors general. They had personal ties to the director general, they were totally loyal to the director general, but not necessarily to the organization."

Mr. Matsuura said he immediately relocated the special advisers to the basement and, since they were on one-year contracts, all have now left. Senior staff members also numbered 200, 90 more than the approved limit. He brought this number below 110 by demoting some officials and encouraging others to leave. "About 20 had been given last-minute promotions," he said. "I formed an evaluation committee which concluded that none deserved promotion."

The shake-up prompted protests, with one group of middle-ranking officials organizing a 10-day hunger strike early in 2000. "But I didn't give in," Mr. Matsuura added.

Some governments even approached him to defend the positions of officials who had been demoted. But he insisted that recruitment would now take place through what he called "a transparent and democratic process."

Among new senior staff members are Pierre Sané, former secretary general of Amnesty International; Sir John Daniel, former head of Britain's Open University; and Abdul Waheed Khan, former rector of Indira Gandhi Open University in India. By next year, when the United States should have completed its re-entry process, which includes a payment of $60 million, Americans will also be sought for recruitment.

But now Mr. Matsuura wants to draw attention to what Unesco is doing in the field, notably its work in promoting literacy, scientific exchanges, press freedom and protection of cultural heritage.

"We need to produce concrete and visible results," he said. "We organize seminars, but that is not enough. We have to be present in education, bioethics, cultural diversity and intercultural dialogue."