RNA from bromegrass mosaic virus was isolated by treatment of the virus with phenol in the presence of bentonite and sodium dodecyl sulfate. The RNA sedimented as three distinct components in the ultracentrifuge. Their sedimentation coefficients, S 20,w 0, in 10 −1 M-KC1 + 10 −2 M-potassium acetate + 10 −3 M-MgCl 2 + 10 −3 M-CaCl 2 (pH 5·5), were 14·0 s, 22·3 s and 26·8 s. Essentially no change in the sedimentation coefficients of the three components or in their relative amounts was observed after incubation at 20°C for 48 hours or after solution in and recovery from formamide. The identical three-component sedimentation pattern was found with RNA isolated by treatment of the virus with 1 M-CaCl 2. RNA isolated from virus variously grown, harvested, purified and treated showed the same pattern. Addition of partially purified host-plant nuclease to the RNA resulted in comparable degradation of each component rather than conversion of the large component into the two smaller components. It was concluded that the three components of bromegrass mosaic virus RNA were not artifacts of virus or RNA isolation but represented the situation in the virus particles themselves in the infected host. The components were separated by sucrose density-gradient centrifugation. Their molecular weights, calculated from the Scheraga-Mandelkern equation, were 3 × 10 5, 7 × 10 5 and 1·0 × 10 6. Base compositions of the three components were similar. It is believed that some bromegrass mosaic virus particles contain the large component whereas others contain the two smaller components. Possibly the two smaller components are pieces of the large component resulting from a single cleavage at some stage in virus synthesis.