The self-assembly of Brome mosaic virus protein has been studied as a function of pH and ionic strength at 22°. At ionic strengths above 0.25, the dissociated protein (dimers of the subunit) polymerized quantitatively below pH 5 into empty capsids or PTC (pseudo top component) in a fully reversible manner. The threshold of reassociation was displaced to higher pH values with increasing ionic strength. Once formed, PTC particles could as well be dissociated into ribbons of protein at low pH, but at low ionic strength only; in the latter configuration, BMV protein was unable to form shells upon reverting to initial conditions. Filamentous structures also formed from dissociated protein when reassociation was performed at low ionic strength; these ribbons too could hardly, if ever, give rise to PTC particles upon shifting to conditions promoting shell formation. Here again, an increase in ionic strength at constant pH resulted in a higher degree of polymerization, suggesting a major role for hydrophobic forces in the formation of organized structures from BMV protein subunits.