Cucumber green mottle mosaic virus (CGMMV) is a rod-shaped virus of the tobacco mosaic virus (TMV) group. The structure of cucumber green mottle mosaic virus has been determined by fiber diffraction methods at 3.4 A resolution, and refined by molecular dynamics methods to an R factor of 0.093. Disassembly of TMV is driven by the mutual repulsion of intersubunit carboxyl-carboxylate pairs, but one of these pairs is not conserved in CGMMV. An alternative pair, located about 5 A from the site of the TMV pair, has been found in CGMMV. Comparison of the two structures suggests that the carboxylate groups are free to migrate in the subunit interfaces during evolution.