Abstract By analysis on SDS-polyacrylamide gels, the coat protein of beet yellows virus was shown to consist of a single polypeptide of molecular weight about 22,400. The amino acid composition and tryptic peptide map also indicate a molecular size close to 22,000. Viral RNA comprising 6% by weight of the particles is single stranded and has the base composition U, 23%; C, 22%; A, 27%; G, 28%. Its size, estimated by sedimentation and gel electrophoresis, is close to 4.6 × 10 6. We propose that in the virus particle the single helical RNA strand is embedded at a radius of 3.3 nm in a protein coat comprised of approximately 3400 protein subunits, giving a total particle weight of 80 million daltons.