Just six weeks before she moves into Mass. Hall, University President-elect Drew G. Faust is scrambling to find a new dean for the Faculty of Arts and Sciences (FAS), according to two individuals close to University administrators.

Faust offered the position to Dean for the Physical Sciences Jeremy Bloxham, but he recently turned it down, according to the two sources and an individual informed of the decision by an FAS administrator, adding uncertainty to the search for a permanent leader of Harvard’s largest school.

With Bloxham out, two of the most serious contenders appear to be Sociology Department Chair Robert J. Sampson and Psychology Department Chair Stephen M. Kosslyn, though other professors could still be candidates, the two individuals close to University administrators said.

Bloxham’s refusal leaves Faust, currently the dean of the Radcliffe Institute for Advanced Study, in a difficult position as she attempts to make one of the most important decisions of her tenure.

In addition to the FAS deanship, Faust must appoint leaders for Harvard Medical School and the Graduate School of Design before taking office on July 1.

A Harvard spokesman could not be reached for comment late last night, but the University has maintained a policy of not commenting on dean searches. Bloxham declined to comment on the search last week, and he and Kosslyn did not return requests for comment yesterday. Bloxham’s decision to decline the deanship was first reported last week by the undergraduate blog Gadfly.

Bloxham, a geophysicist, was considered by several administrators to be an appealing candidate because of his background in the sciences and his experience as one of four FAS divisional deans. As dean of the physical sciences, Bloxham oversees finances, helps with academic planning, and advises the FAS dean on faculty appointments in the physical sciences.

His experience as a scientist was considered a strength of his candidacy as FAS prepares to expand its science faculty and facilities in Allston over the coming years.

Kosslyn joined Harvard’s Psychology Department in 1983. He received his doctorate at Stanford and previously taught at Brandeis University and Johns Hopkins University. His research has focused on cognitive psychology and neuroscience.

Sampson, currently on leave, has served as chair of the Sociology Department for two years. Like Faust, who has only been at the University since 2001, Sampson is a relative newcomer to Harvard. A scholar of crime and neighborhood effects, he spent most of his career teaching at the University of Chicago and was hired by Harvard in 2003.

Physics Department Chair John Huth, who was initially seen as a serious contender for the job, is now considered an unlikely appointment, one of the individuals said.

Theda Skocpol, who is dean of the Graduate School of Arts and Sciences and a frequently mentioned candidate, is no longer in serious consideration, the two individuals said. Skocpol resigned from her deanship at GSAS in March after just two years.

The Crimson granted anonymity to the sources because the individuals’ relationship with University and FAS officials would be compromised if the sources were named.

FAS has lacked a permanent leader since William C. Kirby resigned under pressure from former University President Lawrence H. Summers last year.

The Faculty deanship is considered one of the most demanding positions at Harvard, given the school’s size, its relative lack of high-level administrators to share the burden of decision-making, and the sheer stamina the job requires. The Faculty’s expenses exceeded $800 million in fiscal year 2005, and the school’s endowment of $12 billion tops Princeton University’s.

Jeremy R. Knowles, who served as FAS dean from 1991 to 2001 before reassuming the position last summer on an interim basis, is said to have worked up to 18 hours a day, seven days a week during his first term. Knowles stepped down from hist post in April to take time to recover from complications caused by prostate cancer.

While Faust’s choice is expected to take office in six weeks, past FAS deans have followed a similar timetable.

In 1991, Knowles was appointed just three weeks before he took the helm of FAS, and Summers tapped Kirby for the post in mid-May of 2001.

Earlier this month, Faust appointed computer scientist Barbara J. Grosz, previously mentioned as a candidate for the FAS deanship, to lead Radcliffe as interim dean while Faust conducts a search for a permanent leader.

Other names mentioned as possible candidates for the position include Professor of the History of Science Allan M. Brandt, Government Department Chair Nancy L. Rosenblum ’69, Stanfield Professor of International Peace Jeffry A. Frieden, and Associate Dean for Computer Science and Engineering Michael D. Smith.

Before she takes the helm in July, Faust must also appoint administrators to fill some of the University’s top positions, including a vice president for alumni relations and fundraising to replace Donella M. Rapier, who resigned from the job in February.

—Javier C. Hernandez contributed to the reporting of this story.

—Staff writer Claire M. Guehenno can be reached at guehenno@fas.harvard.edu.