The Smithsonian Institution announced Monday that its top official, Secretary Lawrence M. Small, has resigned amid criticism about his expenses.

Small resigned over the weekend, and the decision was unanimously accepted Sunday by the Smithsonian's Board of Regents.

Cristian Samper, director of the Smithsonian's National Museum of Natural History, has been appointed acting secretary while the regents conduct a nationwide search for a permanent replacement.

An internal audit in January found that Small had made $90,000 in unauthorized expenses, including private jet travel and expensive gifts.

The audit also found that Small charged the Smithsonian more than $1.1 million for use of his home since 2000. The housing expenses included $273,000 for housekeeping, $2,535 to clean a chandelier and $12,000 for service on his backyard swimming pool.

On Thursday, the Senate approved freezing a proposed $17 million increase in funding for the Smithsonian, citing Small's excessive compensation and spending.

Small was to earn $915,698 this year in total compensation — more than double what he earned during his first year as secretary in 2000.

"I think (Small) was really concerned about the possibility of the Institution being changed fundamentally" in regards to the Senate's vote to withhold money, said Roger Sant, chairman of the Smithsonian board's executive committee. "It would be hard to ignore something like that."

Samper, a Harvard-trained biologist who joined the Smithsonian in 2001 as acting director of the Smithsonian Tropical Research Institute in Panama, said in a statement: "My priority in the coming months will be to strengthen the public trust in the Smithsonian Institution and ensure that our priority projects and activities continue on track."

Sant praised Small's seven years as secretary, saying he raised $1 billion for the institution, which he said was a record.

Chief Justice John Roberts, who serves as chancellor of the Smithsonian, also offered praise for Small.

"The Board recognizes that the Smithsonian Institution has benefited greatly from your leadership and dedication," he said in a letter to Small, accepting his resignation.

Small is not entitled to a severance package because he left voluntarily.