Simian rotavirus (SA-11) isolated from infected African green monkey kidney cells was separated into 2 virus fractions in a CsCl density gradient and their proteins analyzed on a continuous phosphate buffered polyacrylamide gel electrophoresis system. One peak (buoyant density 1.37 g/ml) contained double capsid virus particles which were radioimmunoassay (RIA)- and hemagglutinin (HA)-positive and yielded 8 polypeptides whose MW ranged from 48,000-128,000. The 2nd peak (buoyant density 1.39 g/ml) contained 70% single capsid particles, was RIA-positive but HA-negative and yielded only 5 polypeptides. The 3 polypeptides missing in the 2nd peak are associated presumably with the outer capsid of SA-11 virus particles; 1 or more of these are assumed to be the HA of SA-11 rotavirus.