Nuclear physicist Gary L. Smith resigned this week as the CIA's deputy director for science and technology, nine months after assuming the reins of a sprawling directorate still wrestling with its mission in the information age.

CIA Director George J. Tenet announced Smith's resignation Wednesday in a news release and immediately appointed Smith's deputy, Joanne O. Isham, a 23-year CIA veteran with a background in managing reconnaissance projects, to succeed him as head of the Directorate of Science and Technology (DS&T).

Tenet gave no explanation for Smith's sudden departure beyond saying that the scientist wanted to resume his retirement. Smith, 64, joined the CIA after retiring as director of the Johns Hopkins University Applied Physics Laboratory. Smith did not return a call for comment.

One former intelligence official in close contact with DS&T said Smith's resignation did not appear to involve any clash with Tenet or other senior CIA management.

But Jeffrey T. Richelson, an intelligence historian completing a book about the directorate, called Smith's resignation a surprise and said he doubted it was as simple as Tenet described.

"There has to be a bigger story here," Richelson said, noting that the shortest previous tenure for a DS&T director was three years. He also expressed surprise that Tenet named Isham, who is not a scientist, as Smith's successor.

Richelson said the DS&T, credited with developing the U-2 spy plane and the nation's first spy satellites, has been trying to define a new mission in the field of information technology since former head Ruth David resigned in the fall of 1998.

The directorate is still involved in everything from exotic signals intelligence to prosaic foreign press translations. But Richelson said the directorate lost its imagery analysis center in 1996 to the new National Imagery and Mapping Agency (NIMA) and has seen its influence in satellite design diminish at the National Reconnaissance Office (NRO).

Isham, who has a bachelor's degree in government and international studies from Notre Dame, has served as associate deputy director since 1996, having previously headed the CIA's Office of Congressional Affairs.

Mark M. Lowenthal, former deputy secretary of state for intelligence and staff director of the House Permanent Select Committee on Intelligence, praised Isham's appointment and said her lack of a scientific background would not hamper her.

"She understands collection, she understands what the systems are designed to do, and she understands the use to which the intelligence is being put," Lowenthal said.