Sen. Robert C. Byrd was hospitalized with a fever after showing signs of lethargy and sluggishness at home, his spokesman told FOX News Monday.

Legislative aide Jesse Jacobs said the 90-year-old Byrd, the longest-serving U.S. senator, looked ill after casting an evening vote on a climate change bill.

Jacobs said that paramedics were called to the home of West Virginia Democrat and he was later transported to a hospital where doctors diagnosed him with a fever and will hold him overnight for observation.

Aides said that his condition doesn't appear to be serious.

Byrd was hospitalized March 5 for tests after a reaction to antibiotics. A week earlier he was hospitalized at Walter Reed Army Medical Center after a fall at home.

Jacobs said Byrd looked well during the dedication of a new Customs and Border Patrol training center in Harpers Ferry, W. Va. last Friday.

FOX News' Trish Turner and The Associated Press contributed to this report.