In the fall of 1995, a 20-day-old female ostrich chick, 1 of a group of 20, was presented live with clinical signs of 2 days duration characterized by unsteady gait, circling to the left, and walking backward. Another bird with similar clinical signs had died and another had recovered. The bird was euthanized and examined at necropsy. Twenty-five milliliters of serous fluid was in the abdominal cavity and there was increased pericardial fluid. Histopathology of the brain revealed mild to moderate nonsuppurative encephalitis characterized by mild multifocal malacia, perivascular cuffing by lymphocytes, and gliosis. The heart had multifocal infiltrations of lymphocytes mixed with macrophages and a few plasma cells throughout the myocardium. Cytopathic effects were observed in primary chicken embryo liver cells following inoculation with a tissue homogenate prepared from the brain of the affected ostrich. Virus particles the size and morphology of the family Bunyaviridae were observed in cell culture lysate by negative-stain electron microscopy. Viral characterization demonstrated that the virus isolate is a previously unknown serotypic variant (subtype) of Turlock virus. Twelve of 65 sera collected over a 3-year period from ostriches aged from 1 month to 4 years were positive for neutralizing antibody to both the Turlock prototype strain and the new subtype of Turlock virus described in this report.