Cells infected with mammalian reoviruses contain phase-dense inclusions, called viral factories, in which viral replication and assembly are thought to occur. The major reovirus nonstructural protein mu NS forms morphologically similar phase-dense inclusions when expressed in the absence of other viral proteins, suggesting it is a primary determinant of factory formation. In this study we examined the localization of the other major reovirus nonstructural protein, sigma NS. Although sigma NS colocalized with mu NS in viral factories during infection, it was distributed diffusely throughout the cell when expressed in the absence of mu NS. When coexpressed with mu NS, sigma NS was redistributed and colocalized with mu NS inclusions, indicating that the two proteins associate in the absence of other viral proteins and suggesting that this association may mediate the localization of sigma NS to viral factories in infected cells. We have previously shown that mu NS residues 1 to 40 or 41 are both necessary and sufficient for mu NS association with the viral microtubule-associated protein mu 2. In the present study we found that this same region of micro NS is required for its association with sigma NS. We further dissected this region, identifying residues 1 to 13 of mu NS as necessary for association with sigma NS, but not with mu 2. Deletion of sigma NS residues 1 to 11, which we have previously shown to be required for RNA binding by that protein, resulted in diminished association of sigma NS with mu NS. Furthermore, when treated with RNase, a large portion of sigma NS was released from mu NS coimmunoprecipitates, suggesting that RNA contributes to their association. The results of this study provide further evidence that mu NS plays a key role in forming the reovirus factories and recruiting other components to them.