Members of the Paramyxoviridae such as measles, mumps, and parainfluenza viruses have pleomorphic, enveloped virions that contain negative-sense unsegmented RNA genomes. This is encapsidated by multiple copies of a viral nucleocapsid protein N to form a helical ribonucleoprotein complex (termed the nucleocapsid), which acts as the template for both transcription and replication. Structure analysis of these viruses has proven challenging, owing to disordered regions in important constituent proteins, conformational flexibility in the nucleocapsid and the pleomorphic nature of virus particles. We conducted a low-resolution ultrastructural analysis of Sendai virus, a prototype paramyxovirus, using cryo-electron tomography. Virions are highly variable in size, ranging approximately from 110 to 540 nm in diameter. Envelope glycoproteins are densely packed on the virion surface, while nucleocapsids are clearly resolved in the virion interior. Subtomogram segmentation and filament tracing allowed us to define the path of many nucleocapsids and in some cases to determine the number of putative genomes within a single virus particle. Our findings indicate that these viruses may contain between one and six copies of their genome per virion and that there is no discernible order to nucleocapsid packaging.