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The nucleotide sequences ofthe genome RNA species (RNA-1 and RNA-2) of three strains of tobacco rattle virus OR, SYM and CAM, were compared by hybridization with complementary DNA copies. Strain OR was found to contain two RNA-2 species, which differ by about 5 × 104 in mol. wt. (150 nucleotides), but about 90 ~ of whose nucleotide sequences are homologous. The larger of the two contained the genetic information for yellow symptoms in infected Nicotiana spp., whereas the smaller, which probably arose from it by mutation, did not. The RNA-2 species of strains OR, SYM and CAM had no detectable nucleotide sequences in common. RNA-1 of strain OR and of strain SYM were almost identical in sequence, but had no detectable sequence in common with RNA-1 of strain CAM. Only in strain CAM was an