Data are presented which show that bromegrass mosaic virus has a particularly low molecular weight and nucleic acid content. A molecular weight of 4.6×10 6 was calculated from the sedimentation coefficient, S° 20,w = 86.2S, the diffusion coefficient, D 20,w = 1.55×10 -7 cm 2 /sec., and an assumed partial specific volume, [UNK] = 0.708 ml/gm. The virus has a ribonucleic acid content of 1.0×10 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.