A human rotavirus isolate (the 69M strain) with a "super-short" RNA electropherotype has been classified as serotype 8, a new human serotype [Matsuno et al., 1985]RNA-RNA hybridization using 32P-labeled transcription probes was used to assess the genetic relatedness of the 69M strain to a panel of human and animal rotaviruses. The 69M strain showed a medium level of homology with subgroup 1, serotype 2 human rotaviruses with short RNA electropherotypes. However, the 69M strain was not significantly related to any other human rotavirus strains tested in this study, including recently identified serotype 9 strains (represented by the WI61 strain) and subgroup 1 human rotaviruses with long RNA electropherotypes (represented by the AU-1 strain) that are shown to be genetically distinct from other human rotavirus strains. Unexpectedly, a medium level of homology was also found between the 69M probe and the double-stranded RNAs from bovine rotavirus strains (represented by the NCDV strain).