The structural phenotype of the movement proteins (MPs) of two representatives of the Bromoviridae, alfalfa mosaic virus (AMV) and brome mosaic virus (BMV), was studied in protoplasts. Immunofluorescence microscopy showed that the MPs of these viruses, for which there has been no evidence of a tubule-guided mechanism, assemble into long tubular structures at the surface of the infected protoplast. Electron microscopy and immunogold analysis confirmed the presence of both MP and virus particles in the tubules induced by AMV and BMV. The significance of the tubule-forming properties of these viral MPs is discussed.