Loss of infectivity to insect vector cell monolayers of rice dwarf phytoreovirus (RDV) after CCl4 treatment was associated with the removal of one of the viral proteins from the virus particles. This protein, encoded by genome segment S2 and thus named P2 protein, was located at the outer capsid of the virus particle. When RDV was treated with CCl4 for various times, the reduction in the amount of P2 protein and the loss of viral infectivity to vector cell monolayers were proportional to treatment time. RDV purified using CCl4 treatment thus lacking P2 protein, lost the ability to infect vector insects through feeding and consequently failed to be transmitted to plants. These results suggest that P2 protein is essential for RDV infection of the insect cells and may be related to transmission of the virus by the vector insect.