Colloidal gold was used to localize six of the ten known proteins of the simian rotavirus SA11 within infected cells by ultrastructural immunocytochemistry. Monospecific or monoclonal antibodies to selected structural and nonstructural proteins were the primary antisera. The major outer capsid glycoprotein, VP7, was associated with nonenveloped particles, with particles de-enveloped by Triton X-100 and with both nuclear and cytoplasmic inclusions. The protease-sensitive outer capsid protein, VP3, was also found on nonenveloped and de-enveloped particles. The major inner capsid protein, VP6, was accessible to antibodies on some of the nonenveloped particles (presumably single-shelled particles) and on the de-enveloped particles. A monospecific antibody to the gene 11 product, believed to be a precursor to a minor structural protein, VP9, reacted strongly with viroplasmic inclusions. Virus particles were weakly labeled by this antibody. NS35, a nonstructural SA11 protein, was found only in the viroplasms. NS29, a nonstructural glycoprotein, was localized to the cytoplasmic side of the endoplasmic reticulum membrane and to the inside of enveloped virus particles. These data support the hypothesis that NS29 facilitates budding of the virus particles and acquisition of the outer capsid layer.