The genomic RNA of infectious flacherie virus (IFV), an insect picornavirus, was shown to contain a covalently attached substance which can be labeled with 125I-Bolton and Hunter reagent. This substance was found to be a protein covalently linked to the virus genome, because it cosedimented and was copurified with the viral RNA and was degraded by proteinase K but not by RNases. The molecular weight of the protein was found to be 11,500 in the sodium dodecyl sulfate polyacrylamide gel electrophoresis. This protein apparently corresponds to viral protein genome-linked (VPg) detected in mammalian picornaviruses, because it was not required for the synthesis of viral proteins in a cell-free translation system of a rabbit reticulocyte lysate and because it was not immunochemically related to the virus capsid proteins. The results presented here leave a possibility that insect picornavirus also replicates its RNA by using VPg as a primer as reported for mammalian picornaviruses. These observations on VPg of an insect picornavirus are discussed in the view point of comparative virology.