Vesicular stomatitis virus (VSV) has been disrupted with nonionic detergent plus 0.5 M NaCl under conditions which result in solubilization of the viral glycoprotein (G), matrix protein (M), and lipids, leaving the nucleocapsid in a highly extended state. Dialysis of these suspensions to remove NaCl was found to result in reassociation of nucleocapsids with M protein. Reassociated structures were highly condensed and similar in appearance to "native" VSV skeletons produced by extraction of virions with detergent at low ionic strength. For instance, electron microscopic analysis revealed that, like "native" skeletons, "reassembled" skeletons were cylindrical in shape, with diameters in the range of 51.0 to 55.0 nm and cross-striations spaced approximately 6.0 nm apart along the length of the structure. Like native skeletons, reassembled skeletons were found by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to contain the viral N and M proteins, but they lacked the glycoprotein entirely. Both native and reassembled skeletons were found to be capable of in vitro RNA-dependent RNA synthesis (transcription). In vivo skeleton assembly required the presence of M protein and nucleocapsids. No skeleton-like structures were formed by dialysis of nucleocapsids in the absence of M protein or of M protein in the absence of nucleocapsids. These results provide strong support for the view that the VSV M protein plays a functional role in condensing the viral nucleocapsid in vitro and raise the possibility that it may play a similar role in vivo.