Group B rotaviruses (GBRs) were recognized recently as causative agents of gastroenteritis. Investigations into the relatedness of various heterologous GBR strains have been hindered by the difficulty of growing these viruses in cell culture. Viral RNA extracted from experimentally infected rats used to prepare cDNA clones. From these, the nucleotide sequences of genes 9 and 11 of the IDIR strain of GBR were determined and compared with the corresponding sequences of the human ADRV strain of GBR. IDIR gene 11 is 643 bp in length with a single open reading frame (ORF) encoding 174 amino acids; IDIR gene 9 is 804 bp in length with a single ORF encoding 246 amino acids. Comparison of the IDIR sequences with those of ADRV showed that nucleotide sequence similarity was 60.6% and 71.9% for genes 9 and 11, respectively. The deduced amino acid sequence similarity was 51.2% for the gene 9 and 66.5% for the gene 11 product. This sequence diversity indicates that GBRs are more distantly related than strains of group A rotavirus.