Among members of the genus Orthoreovirus, family Reoviridae, a group of non-enveloped viruses with genomes comprising ten segments of double-stranded RNA, only the "non-fusogenic" mammalian orthoreoviruses (MRVs) have been studied to date by electron cryomicroscopy and three-dimensional image reconstruction. In addition to MRVs, this genus comprises other species that induce syncytium formation in cultured cells, a property shared with members of the related genus Aquareovirus. To augment studies of these "fusogenic" orthoreoviruses, we used electron cryomicroscopy and image reconstruction to analyze the virions of a fusogenic avian orthoreovirus (ARV). The structure of the ARV virion, determined from data at an effective resolution of 14.6 A, showed strong similarities to that of MRVs. Of particular note, the ARV virion has its pentameric lambda-class core turret protein in a closed conformation as in MRVs, not in a more open conformation as reported for aquareovirus. Similarly, the ARV virion contains 150 copies of its monomeric sigma-class core-nodule protein as in MRVs, not 120 copies as reported for aquareovirus. On the other hand, unlike that of MRVs, the ARV virion lacks "hub-and-spokes" complexes within the solvent channels at sites of local sixfold symmetry in the incomplete T=13l outer capsid. In MRVs, these complexes are formed by C-terminal sequences in the trimeric mu-class outer-capsid protein, sequences that are genetically missing from the homologous protein of ARVs. The channel structures and C-terminal sequences of the homologous outer-capsid protein are also genetically missing from aquareoviruses. Overall, the results place ARVs between MRVs and aquareoviruses with respect to the highlighted features.