Andrew S. Natsios, the longtime chief of the U.S. Agency for International Development, announced yesterday that he is resigning to join Georgetown University's Edmund A. Walsh School of Foreign Service.

Natsios traveled widely and oversees an agency that in 2005 distributed about $9 billion in development aid and an additional $3 billion in emergency aid to cope with earthquakes, tsunamis and other disasters.

But some critics have said he has allowed USAID to be shoved aside from its central role in development as the administration created a new program, known as the Millennium Challenge Accounts, that aids countries that meet certain political and economic goals.

Jim Wilkinson, senior adviser to Secretary of State Condoleezza Rice, said she tried to convince Natsios to stay. "This is very bittersweet for me personally and for the United States government," Rice said, announcing the resignation. She said Natsios had had a "tremendous impact."

Natsios said the offer from Georgetown, which he and his son attended, was "too tempting for me."

Natsios will join Georgetown in January as distinguished professor in the practice of diplomacy and adviser on international development.

Natsios has headed USAID since May 2001. He had also served at USAID as director of the Office of Foreign Disaster Assistance from 1989 to 1991 and then as assistant administrator for the Bureau for Food and Humanitarian Assistance from 1991 to January 1993.

Before joining the Bush administration, Natsios was chairman and chief executive officer of the Massachusetts Turnpike Authority, where he managed the Central Artery/Tunnel Project, or "Big Dig." He also served 12 years in the Massachusetts House of Representatives.

Andrew S. Natsios, head of USAID, said Georgetown's offer was "too tempting."