Purified preparations of lucerne mosaic virus (LMV) contained bacilliform particles, which, mounted in neutral potassium phosphotungstate, were 18 mμ wide and of various lengths though mostly 36 mμ, 48 mμ, and 58 mμ long, with sedimentation constants ( S 20) of 73, 89, and 99, respectively. Old preparations contained many particles 20–30 mμ long. Particles of all sizes contained about 17% nucleic acid, were serologically and electrophoretically indistinguishable, and, except for the 20–30 mμ particles, seemed to be equally infective. Negatively stained preparations showed evenly spaced knobs protruding from the surface of the particles; the arrangement of these knobs suggests that LMV has a structure quite unlike that of any plant virus so far described.