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RNAs of soil-borne wheat mosaic virus (SBWMV) from virions 281 nm, 138 nm and 92 nm long (designated here by relative lengths as 1∙0L, 0∙SL and 0∙35L, respectively), were isolated and purified by three cycles of sucrose density gradient centrifugation. Infectivity assays with these RNAs proved the bipartite nature of SBWMV, the combination of 1∙0L and either 0∙5L or 0∙35L RNAs being required for infection and for multiplication of progeny viruses. The 0∙5L RNA underwent deletion mutation, producing smaller variants with various sizes, of which 0∙4L and 0∙35L RNAs were confirmed to be functional in combination with 1∙0L RNA. The coat proteins of all isolates had mol. wt. of 19700. The mol. wt. of 1∙0L, 0∙5L, 0∙4L and 0∙35L RNAs, determined under denaturing conditions, were 2∙28 × 106 (6500 bases), 1∙23 x 106 (3500 bases), 0-97 x 106 (2800 bases) and 0∙86 x 106 (2450 bases), respectively. A new virus group, furovirus (fungus-borne rod-shaped virus), is proposed for SBWMV.