Rodlike tobacco mosaic virus (TMV) has been found to assemble into a 2D superlattice in aqueous solution with hexagonally packed structures in the presence of Ba2+ through like-charge attraction whereas lower-Z divalent ions such as Zn2+, Cd2+, Mg2+, and Ca2+ induce only liquidlike ordering. The molar ratio between Ba2+ and TMV is a crucial parameter in the formation of the superlattice. There is a critical molar ratio of Ba2+ to TMV at which TMV exhibits a transition from a nonordered colloidal state to an ordered crystalline state. It is also found that the superlattice is formed regardless of the pH and TMV concentration within the range studied.