We used DNA nanotubes obtained by the thermal annealing of 5 short DNA strands (500 nM each, including a fluorescently labelled one) in a so called TAMg buffer (Trizma base 40 mM, acetic acid 20 mM, MgCl2 12.5 mM). We hypothesized that the addition of multivalent cations capable of DNA condensation would induce the formation of superstructures that could be disassembled with a further addition of monovalent ions in a sufficiently large excess (Fig. ). Initial nanotubes appeared as elongated filaments freely fluctuating in solution as observed by fluorescence microscopy (Fig. left, top image and Movie S1). Transmission electron microscopy (TEM) revealed that they were well individualized (Fig. left, bottom image) with a uniform diameter of 12 ± 3 nm (Fig. ), in agreement with previous reports. We first added spermine (noted SPM 4+ ), a natural tetraamine, and observed the resulting structures in bulk by fluorescence microscopy. Addition of millimolar amounts of SPM 4+ led to immediate formation of arrested structures of large dimensions floating in solution, with a low fluorescence background indicating that most nanotubes were engaged in these clustered structures (Fig. right, top image and Movie S1). Similar assemblies were observed in the past using high concentrations of Mg 2+ or crowding agents and will be referred to as "bundles". Moreover, TEM revealed a local organization into thick elongated structures (Fig. right, bottom image) with an average diameter of 63 ± 26 nm (Fig. ) where individual nanotubes could not be distinguished anymore, indicating a very strong attraction mediated by SPM 4+ . Interestingly, TEM revealed that individual nanotubes could be distinguished and were found to locally organized into aligned assemblies (Fig. middle, bottom image), showing inter-tube attraction, but without strongly condensing as in the bundles.