Data are presented which show that bromegrass mosaic virus has a particularly low molecular weight and nucleic acid content. A molecular weight of 4.6 x 10(6) was calculated from the sedimentation coefficient, S degrees (20,w) = 86.2S, the diffusion coefficient, D(20,w) = 1.55 x 10(-7) cm(2)/sec., and an assumed partial specific volume, [UNK] = 0.708 ml/gm. The virus has a ribonucleic acid content of 1.0 x 10(6) atomic mass units. Electrophoresis experiments showed that the virus is stable in 0.10 ionic strength buffers in the pH range 3-6. Breakdown of the virus was observed outside this pH range. Some characteristics of the breakdown products are described.