In April, 1975, an epidemic of human encephalitis was detected In several counties in the State of Sao Paulo, Brazil; the epidemic continued into 1976. A virus was Isolated from central nervous system (CNS) tissues of a 39-year-old male who died on December 8, 1975; the virus was found to be a new flavlvlrus for which the name Rocio virus is proposed. Nine further isolations of Rocio virus were obtained from CNS tissues of 17 patients who died with clinical symptoms of encephalitis. Isolations of virus and serologic evidence of Rocio virus infection In a significant proportion of the encephalitis patients suggested that Rocio virus was the etiologic agent of the epidemic. Rocio virus was Isolated only from patients who died within 5 days of onset of Illness. The virus was Isolated from two sentinel mice exposed In the epidemic zone and from a rufous collared sparrow (Zonotrichia capensis) collected in the area.