A serologic survey of the blacktail jackrabbit ( Lepus californicus ) for infections with 10 arboviruses was conducted from 1971 through 1974 along the Sacramento River in Butte County, California. Of 325 animals captured and bled a total of 493 times, 40% were found positive for hemagglutination-inhibiting (HI) antibody to California encephalitis (CE) virus, 34% to western equine encephalomyelitis (WEE) virus, 20% to Buttonwillow virus, 15% to St. Louis encephalitis (SLE) virus and 12% to Main Drain virus. Only 5 and 2% of the animals had HI antibodies to Lokern and Turlock (TUR) viruses, respectively. There was no serologic evidence for infection of animals with Powassan, Modoc and Rio Bravo viruses. Differenles in monthly and yearly activities of some viruses were found by analyses of lata on antibody prevalence rates and immunologic conversions in recaptured animals. Experimental studies revealed that subadult jackrabbits were highly susceptible by subcutaneous inoculation to infection with CE, WEE and SLE viruses but were refractory to infection with TUR virus. All animals infected with CE and WEE viruses developed viremia that persisted for 2 or 3 days after inoculation and then developed antibodies that were detectable from 7 through at least 56 days after infection. In contrast, only 2 of 7 animals that developed HI antibodies to SLE virus had viremia, and at barely detectable levels; and HI antibodies were undetectable in 3 of the 7 animals at 56 days after infection.