Bean plants infected with the cowpea strain of TMV (B-TMV) have been found to contain two size-classes of virus particles. The long particles are typical TMV rods, approx 300 nm long, and contain RNA of molecular weight 2 × 10 6. The short particles are rods, approx 40 nm long, and contain RNA of molecular weight 0.3 × 10 6 ± 20,000. Base composition analysis of the two RNA species showed minor differences in the adenylic and uridylic acid content. Electrophoresis of the coat-protein of B-TMV on 14% polyacrylamide gels in the presence of sodium dodecyl sulfate revealed the existence of two components of molecular weights approx 18,000 and 16,500. Protein derived from long particles was composed mainly of the larger component whereas protein derived from short particles was composed largely of the 16,500 molecular-weight component. In double diffusion tests, however, using antiserum prepared against a mixture of the proteins, the proteins of short and long particles gave reactions of identity. Tryptic peptide maps of the proteins from the short and from the long particles confirmed that the two proteins were basically the same, the smaller component apparently lacking at least one peptide present in the 18,000 molecular-weight component. Short and long particles were also found in association with B-TMV infections of tobacco plants. Short particles could not be detected in tobacco plants infected with the common strain of TMV. The ratio of the number of short particles to the number of long particles was 1:1 to 2:1 for bean or tobacco plants which had been inoculated 4–6 weeks previously. Purified RNA from short particles (0.3 × 10 6 daltons) was not infectious when inoculated onto bean plants. Long particle RNA, purified through sucrose and formamide-sucrose density gradients, produced symptoms on inoculated bean plants typical of B-TMV infections. Virus preparations extracted from such plants contained short as well as long particles. The possible significance of the short particles in virus infection is discussed in the light of the observation that the short RNA contains the cistron for the viral coat-protein.