Strains of virus belonging, according to the results of electron microscopic examinations, to the herpes virus family, were isolated in Vero cell culture from the organs and aphtha secretions in the buccal cavity of yaks developing the disease during an epizootic (more than 500 animals) in Khubsugul aimak (Mongolian village) in Mongolia, 1990. Virus-neutralizing antibodies were detected in the blood sera of the sick animals. The isolated strains differ from herpes viruses affecting man. No infection could be induced by materials of the sick animals in cows and oxen of different breeds in Mongolia. Seroepidemiological surveys are planned because of the mass character of the disease and its repeated occurrence.