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The sedimentation coefficient of virion I of soil-borne wheat mosaic virus was estimated to be 219S, the same, within error, as the sedimentation coefficient of the dimer of virion II, estimated to be 218S. The monomer of virion II sedimented at 177S, and was 138 nm long. Another strain of the virus had a virion II (designated lIb) that was 92 nm long and sedimented at 159S. The two virus strains coexist in some fields.