We have modeled the arrangement of the RNA within the EBOV nucleocapsid (Table S2 ). Since no atomic resolution data on the ribonucleoprotein structure of filoviruses have yet been reported, the ribonucleoprotein ring-structures of other members of the mononegovirales are useful for comparison. Although the mass of the nucleoproteins, and the number of nucleotides per subunit differs amongst these virus families, a model of the 18.9 kb EBOV genome with the RNA following a circular fixed radius as in respiratory syncytial virus (RSV) [32] would make a nucleocapsid containing one copy of the EBOV genome about 914 nm long (Table S2 ). This fits closely with the 982 nm length class (53% of the particles observed in this study) containing a single copy of the EBOV genome threaded through the NP at a fixed radius. Allowing 33 nm of space for the membrane envelope to curve around at each end of the virus particle gives an estimated length for a single-genome virus particle of 980 nm, which is very close to that observed (982679 nm). The majority of the virus particles fall into size categories that are a multiple of the putative single genome length (G1), giving size classes of 1.960.15 mm (G2: 18.7% of the particles having 2 genomes); 2.960.2 mm (G3: 12% of particles having 3 genomes) and so on ( Figure 1A and Table S1 ) . The length of the longest particle measured was consistent with having 22 genome copies. Our model predicts a nucleotide to NP ratio of 13.0 (Table S2) , which is within the range of 12 to 15 nucleotides per NP molecule as measured by biochemical studies of Marburg virus [33] . In partially full virions, the membrane envelope is constricted at the transition point where the nucleocapsid ends and the empty membrane tube begins (Figures 1, S1C ). Empty virions ( Figure  S1C ) have a similar structure to VP40-GP virus-like particles ( Figure 3A ). Both full and empty virions have a continuous layer of spikes projecting from the surface (Figures S1, S2, S3, movie S2), giving an overall diameter of 120 nm for full particles. The majority of virions are linear (Figures 1, S1A, S5 ), others have a ''comma-shaped'' appearance, with a globular head containing portions of the nucleocapsid that are curled-up or bent at one end ( Figures 1A,B and S1B). It is clear that ''toroidal'' virions previously identified by negative staining [22, 34] are a variation of comma-shaped virions. Internal vesicles of 20-40 nm in size are also sometimes observed at the ends of virions ( Figure S1D ). These vesicles appear to be formed during the process of envelopment of the nucleocapsid, since they were not observed in preparations of VP40 or VP40-GP VLPs. VP40 VLPs had wavy envelopes with an irregular diameter ranging between 48 nm and 142 nm (N = 49) compared to VP40-GP VLPs which were more ordered with a diameter between 50 nm and 91 nm (N = 26), (Figure 3 ). Thus the presence of GP, and certain contacts between the GP and VP40, play a part in stabilizing the tubular membrane envelope structure, and our observed structure agrees with previous reports that GP enhances VP40 VLP budding [35, 36] . The previously reported ''branched'', filamentous forms [34] were rarely observed: these consist of empty tubes (data not shown). It is possible that centrifugal virus purification disrupted most branched structures that were seen previously with negative staining of cell culture supernatant.