The three-dimensional structures of double and single-shelled simian rotavirus have been determined to a resolution of 40 A by image processing electron micrographs of unstained, unfixed virus particles embedded in vitreous ice. This study demonstrates that the icosahedral surface lattices in these structures have a triangulation number of 13 in a left-handed configuration. The double-shelled virion has a smooth outer surface with 60 slender spikes. The single-shelled virion, in contrast, exhibits a bristly surface. On the basis of these structures, the locations and number of copies of outer and inner shell proteins have been deduced. The spikes likely correspond to VP3, a hemagglutinin, while the rest of the mass density in the outer shell represents 780 molecules of VP7, a neutralization antigen. The 260 morphological units, located on all the local and strict 3-fold axes of the single-shelled virion are proposed to represent 260 trimers of VP6, which is a subgroup antigen. The regions of closed contact between the outer and the inner shells are located mainly near the local and strict 3-fold axes. A distinctive feature in the rotavirus structure is the presence of 132 large channels spanning across both the shells at all 5 and 6-co-ordinated positions linking the outermost surface with the inner core. In the transcriptionally active single-shelled virion, these channels may provide pathways for importing the metabolites required for the viral RNA transcription and exporting the newly synthesized RNA molecules for subsequent viral replication processes.