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Beet soilborne mosaic virus (BSBMV) is a rigid rod-shaped virus transmitted by Polymyxa betae. Particles were 19 nm wide and ranged from 50 to over 400 nm, but no consistent modal lengths could be determined. Nucleic acids extracted from virions were polyadenylated and typically separated into three or four discrete bands of variable size by agarose-formaldehyde gel electrophoresis. RNA 1 and 2, the largest of the RNAs, consistently averaged 6.7 and 4.6 kb, respectively. The sizes and number of smaller RNA species were variable. The molecular mass of the capsid protein of BSBMV was estimated to be 22.5 kDa. In Northern blots, probes specific to the 3´ end of individual beet necrotic yellow vein virus (BNYVV) RNAs 1–4 hybridized strongly with the corresponding BNYVV RNA species and weakly with BSBMV RNAs 1, 2, and 4. Probes specific to the 5´ end of BNYVV RNAs 1–4 hybridized with BNYVV but not with BSBMV. No cross-reaction between BNYVV and BSBMV was detected in Western blots. In greenhouse studies, root weights of BSBMV-infected plants were significantly lower than mock-inoculated controls but greater than root weights from plants infected with BNYVV. Results of serological, hybridization, and virulence experiments indicate that BSBMV is distinct from BNYVV. However, host range, capsid size, and the number, size, and polyadenylation of its RNAs indicate that BSBMV more closely resembles BNYVV than it does other members of the genus Furovirus.