During a surveillance study (2003–2005) in a cattle market in Kolkata city, state of West Bengal, Eastern India, 34 (13·0%) of 260 calves with diarrhoea were positive for group B rotaviruses (GBR) by RNA electrophoresis in polyacrylamide gels. Analysis of the partial VP7 gene sequence of 28 of the 34 GBR strains revealed maximum identities (97·7–99·5% at nucleotide level and 97·8–100% at amino-acid level) with the novel bovine GBR ‘Kolkata strains’ reported in an earlier surveillance study (1·5%, n=192, 2001–2002) from the same cattle market, and shared low identities of 73·7–78·9% and 80·8–89·6%; 62·6–66·2% and 59·8–65·4%; 58·9–62·2% and 48·6–54·9% at nucleotide and amino-acid level with other bovine, human, and murine GBR. The GBR-infected calves were traced to districts in neighbouring states of West Bengal. Therefore, the present study reports a rapid increase in prevalence (13·0% in 2003–2005 against 1·5% in 2001–2002) of novel GBR strains among calves with diarrhoea, and provides evidence for interstate transmission of GBR.