It is known that electrostatic condensation of a semi-flexible polyelectrolyte like double-stranded DNA (dsDNA) leads to the formation of toroids with a diameter (≈ 100 nm) around twice its persistence length (50 nm), inside which dsDNA double-helices are parallelly packed in a nearly crystalline manner with an interspacing of about 2.4 nm. We thus scrutinized in more detail the regime of bundle formation ([SPM 4+ ] = 2.5 mM). First, we used cryo-electron microscopy (cryo-EM) to reveal the internal structure of the DNA assemblies before and after bundle formation (Fig. ). Without SPM 4+ , we could distinguish the DNA strands in the repeating tile motif forming hollow nanotubes with a diameter in agreement with TEM observations. Notably, after SPM 4+ addition, hollow nanotubes could not be distinguished anymore. Instead, the inner structure of the bundles revealed a tightly packed parallel DNA arrangement with an interspacing of around 3 nm, i.e., a value close to a double-helix diameter (2 nm). This feature, structurally reminiscent to the internal organization of condensed DNA in toroids, confirms the role of SPM 4+ to induce electrostatic condensation of the nanotubes. Moreover, using TEM we detected that the condensed nanotubes also existed in the form self-closed structures similar to DNA toroids but of much larger dimensions (Figs. ). These rings were also observed by confocal microscopy coexisting with bundle fragments adsorbed on the microscopy cover slip surface (Fig left, white arrows). Using super-resolution fluorescence imaging allowing a 120 nm lateral resolution, rings appeared with a single dense contour (Fig. right), in agreement with cryo-EM (Fig. ) and TEM (Fig. ) data, confirming the compact longitudinal assemblies of the nanotubes in these structures. The ring diameter was found to typically vary between 1 and 10 µm (Fig. ), with a mean ± SD value of 2.0 ± 1.7 µm (Fig. ), which is smaller yet of the order of twice the persistence length of nanotubes (≈ 4 µm) . It is also in agreement with a recent report using a large synthetic star-shaped multicationic crosslinking agent. Our results thus show that electrostatic DNA condensation by counter-ion condensation is enough to generate the spontaneous formation of rings and, notably, does not require specific crosslinking interactions. To both understand the physico-chemical mechanisms underlying the formation of the different superstructures and analyse how general this behaviour could be, we established a diagram reporting the nature of the obtained superstructures for different multivalent cations added to DNA nanotubes. The diagram was plotted as function of both the condensing agent concentration (Fig. ) and the charge ratio ρ (Fig. ), which was defined as the concentration of charges brought by the condensing agent normalized by that of DNA (Text S1). Starting with spermine (SPM 4+ ), we observed the formation of networks from [SPM 4+ ] = 0.2 mM (ρ = 7.14) to 0.4 mM (ρ = 14.3) (Fig. ). For larger concentrations, nanotubes were organized into bundles coexisting with a fraction of rings, in agreement with Figs. .