Several plant viruses with filamentous particles ranging in modal lengths from 0.48 µ to 1.25 µ were negatively stained with uranyl formate, examined in the electron microscope, and the electron micrographs analysed in various ways. The particles of all the viruses were helically constructed with a basic pitch of 33 to 37 Å (mean 34 Å), but could be separated into groups by other features of their particles. Various measurements of the particles of five of the viruses suggest that there were 10 to 14 subunits in each turn of the basic helix of their particles.