Isolates of ³H-labelled messenger RNA of a number of different bluetongue virus serotypes were hybridized with saturating amounts of denatured ³²P-labelled double-stranded RNA of different serotypes. These cross-hybridization products were then analysed by polyacrylamide gel electrophoresis. The results indicate relatively large differences between the various serotypes. Only a few of the genome segments in the different serotypes were completely homologous. Each of the cross- hybridization patterns obtained using the genome of Serotype 10 and any one of the other serotypes was unique and characteristic for the strain under investigation. The patterns furthermore clearly indicated different degrees of homology between the genomes of the different serotypes. The immunological specificity of the serotypes appears to be determined mainly by the second genome segment of the virus while genome segment six could be of secondary importance. These results were supported by a study of the cross-hybridization patterns between different isolates of Serotype 4. Cross-hybridization experiments between virulent and attenuated strains of the same serotype also indicated small differences. In all the serotypes investigated the process of attenuation involved changes in genome segments two and six. This result would tend to implicate the same genome segments in the determination of both the immunological specificity and the virulence of the virus.