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The wild-type (WT) isolate of soil-borne wheat mosaic virus has two species of rodshaped virions 281 nm and 138 nm in length, which are designated as virions 1∙0L and 0∙5L, respectively. We reported previously that virions shorter than 0∙5L arose in assay plants inoculated with separated and recombined 1∙0L RNA and 0∙5L RNA and suggested that these short virions were caused by deletion mutation of 0∙5L RNA. We now report that these short virions arose after a period of several months in wheat plants that had been inoculated manually with unpurified WT isolate, and also when plants infected naturally in fields infested by the fungal vector, Polymyxa graminis, were grown at 17 °C. The sizes and relative proportions of virions shorter than 0∙5L varied both from plant to plant and in the same plant sampled at different times. This indicates that the virions shorter than 0∙5L arose by continued and spontaneous deletion mutation of 0∙5L RNA.