Hantavirus pulmonary syndrome is a newly recognized rodent-borne zoonosis. We report a case of hantavirus pulmonary syndrome in an employee of a California utility company who was probably occupationally exposed to Sin Nombre virus. Environmental assessment and genetic comparison of the patient's hantavirus isolates to hantavirus isolates from rodents trapped at possible sites of exposure suggested that the patient contracted his infection at the work site. The study revealed a close correspondence between the patient's viral genotype and that from a rodent trapped at the work site. This report alerts the public health and medical community to the fact that employees of utility companies and similar industries may be an important risk group in areas where hantavirus is endemic and emphasizes the need to incorporate strategies for preventing exposure to hantavirus and other emerging infections into occupational safety protocols.