Summary Two strains of bluetongue virus (BTV) serotype 11, UC-2 and UC-8, were identified by the electrophoretic migration pattern of their genomic RNA segments on polyacrylamide gel electrophoresis. Significant differences in virulence of these two viruses could be demonstrated by subcutaneous inoculation of newborn mice. No signs of disease were observed in mice infected with UC-2. Mice infected with UC-8 died of a severe necrotizing encephalitis, which resembled lesions in bovine and ovine foetuses infected with BTV.