The double-stranded RNA bacteriophage phi6 contains a nucleocapsid enclosed by a lipid envelope. The nucleocapsid has an outer layer of protein P8 and a core consisting of the four proteins P1, P2, P4 and P7. These four proteins form the polyhedral structure which acts as the RNA packaging and polymerase complex. Simultaneous expression of these four proteins in Escherichia coli gives rise to procapsids that can carry out the entire RNA replication cycle. Icosahedral image reconstruction from cryo-electron micrographs was used to determine the three-dimensional structures of the virion-isolated nucleocapsid and core, and of several procapsid-related particles expressed and assembled in E. coli. The nucleocapsid has a T = 13 surface lattice, composed primarily of P8. The core is a rounded structure with turrets projecting from the 5-fold vertices, while the procapsid is smaller than the core and more dodecahedral. The differences between the core and the procapsid suggest that maturation involves extensive structural rearrangements producing expansion. These rearrangements are co-ordinated with the packaging and RNA polymerization reactions that result in virus assembly. This structural characterization of the phi6 assembly intermediates reveals the ordered progression of obligate stages leading to virion assembly along with striking similarities to the corresponding Reoviridae structures.