Hantavirus cardiopulmonary syndrome (HCPS) is a frequently fatal viral disease transmitted through rodent secretions and excretions. Working around deer mice can increase risk of infection. This study assessed potential risk of HCPS at facilities occupied by the US Forest Service (USFS) in California. In 2004-2005, 18 USFS facilities in eight National Forests in California were evaluated for evidence of rodent infestation and circulation of hantavirus. Structural deficiencies and evidence of rodent infestation were observed at 18 facilities. Serum antibodies to hantavirus were detected in 50 of 255 deer mice collected from 15 facilities. Seroprevalence was higher at elevations > 1,600 m (22%). Employees at 14 facilities had received training in rodent-borne disease prevention. Risk of HCPS among USFS employees should motivate inclusion of disease prevention information into employee safety training.