A virus (designated the Shintoku strain) which was morphologically indistinguishable from group A rotaviruses was detected in the feces of adult cows with diarrhea in Japan. The virus contained 11 segments of double-stranded RNA and had an electrophoretic migration pattern in polyacrylamide gels similar to that of other group C rotaviruses (4-3-2-2). Feces containing the bovine virus reacted with antiserum to porcine group C rotavirus (Cowden strain) but not group A or B rotaviruses in immunoelectron microscopy. The virus was adapted to serial propagation in roller tube cultures of a rhesus monkey kidney cell line (MA104) by using high concentrations of trypsin. Evidence for viral replication in MA104 cell cultures was demonstrated by immunoelectron microscopy and indirect immunofluorescence by using antiserum to porcine group C rotavirus and by electrophoretic analysis of extracted viral double-stranded RNA. A significant antibody response against the isolate was detected in convalescent-phase sera of cows which excreted the virus: no increased antibody response to bovine group A rotavirus was observed. To our knowledge, this is the first isolation of a group C rotavirus from cattle.