Serum samples from 60 people diagnosed in the summer of 1989 as having acute respiratory infections, meningitis or meningoencephalitis, acute pneumonia, Lyme disease, or other illnesses were tested by neutralization and hemagglutination-inhibition with Tahyna, Inkoo, and snowshoe hare viruses of the California serogroup (family Bunyaviridae, genus Bunyavirus). Demonstration of a diagnostic increase in antibody titers between paired serum samples from seven people showed that their illnesses were probably caused by Inkoo or Tahyna viruses. The clinical pictures were less specific. One patient had signs of meningoencephalitis, three had signs of meningitis, and three had influenza-like illnesses. These observations and other results suggest that California serogroup viruses are widespread in Russia and may account for an unexpectedly large number of presently undiagnosed febrile illnesses, some with central nervous system involvement.