The rakkyo strain (TMV-R) and the common strain (TMV-U1) of tobacco mosaic virus exhibit distinct host range differences. TMV-R infects rakkyo plants, a monocot host that TMV-U1 is unable to infect. However, TMV-R causes only latent infection in Nicotiana tabacum cv. Bright Yellow (BY) in inoculated leaves, whereas TMV-U1 infects BY systemically and induces mosaic symptoms. Complete nucleotide sequencing of the TMV-R genomic RNA revealed amino acid changes in the 130K/ 180K replicase proteins, the 30K protein, and the coat protein and nucleotide changes in the 5' and 3' noncoding regions compared to TMV-U1. To identify viral components involved in determination of differences in host range, we have mapped determinants for the differential infection phenotype in BY plants by constructing chimeric viruses between the two strains in the present study. Examination of the infection phenotypes of the 14 constructed chimeric viruses in BY showed that determinants defining the differential infection phenotype in BY reside in the 130K/180K replicase proteins and the 3' noncoding region. Cognate combination of the 130K/180K replicase proteins and the 3' noncoding region of TMV-U1 origin is required to produce systemic infection in BY plants.