Recombinant plasmids containing inserts representing genome segments 2, 5, 6, 8, and 9 of bluetongue virus (BTV) serotype 11, with tentative coding assignments for viral proteins P2, P5, NS1, NS2, and P6, respectively, have been used to study the genetic diversity within a BTV serotype using Northern blot hybridization. BTV 11 strains were isolated in California, Nevada, Oklahoma, and Mexico from elk, deer, and cattle. Diversity was indirectly indicated in the BTV 11 strains by comparisons of electropherotypes. Probes specific for segments 2, 6, 8, and 9 hybridized to all BTV 11 strains with only minor variation in hybridization signal. cDNA clones, representing 90 and 20% length copies of gene segment 5, detected a difference in the field isolates with hybridization signal correlating to mobility of this segment in SDS-PAGE. These two cDNA probes of genome segment 5 hybridized to BTV U.S. prototypic serotype 17 and not the remaining serotypes (BTV 2, 10, 13) or epizootic hemorrhagic disease virus (EHDV). These data suggest a close relationship between BTV-11 and 17 and/or alternatively are the result of genome segment reassortment between serotypes.