Cellulose is the main constituent of plant cell walls, is abundant in nature, and is known for its excellent biocompatibility, thermal and mechanical properties.[1] Similarly to polypeptides and DNA, cellulose can form cholesteric liquid crystalline (LC) phases.[2] These phases, with an internal periodic modulation of the refractive index, exhibit many remarkable optical properties as a result of their photonic band structure, which have applications such as polarized light sources, information displays, storage devices, photocopy, and decorative materials.[3] Since 1976, when it was first reported that a cholesteric LC phase could be obtained in aqueous hydroxypropyl cellulose,[4] many cellulose derivatives have been found that exhibit both lyotropic and thermotropic cholesteric LC phases,[5] with particular prominence of acetoxypropyl cellulose (APC). The shear-induced cholesteric to nematic transition in films prepared from lyotropic cellulosic mesophases has also been investigated.[6] Electrostatic fiber spinning or ‘‘electrospinning’’is a process for drawing fibers with sub-micrometer diameters through the action of electrostatic forces.[7, 8] The nonwoven mats thus produced are finding applications in filtration, protective clothing and biomedicine.[8–10] Cellulose and cellulose derivatives have been successfully electrospun into fibers; different cellulose derivatives as well as electrospinning parameters have been systematically investigated.[11–13] In this paper we report the first observation of a new conformational effect taking place during the electrospinning of cellulose, when spun from solutions in the LC phase: the fibers adopt a characteristic helically twisted form. The twisting is on a supramolecular scale, with the pitch measured in micrometers. The helical morphology is similar to what has been seen in amyloid nanofibrils of aggregated peptides [14] and natural cellulose fibrils extracted from algae;[15] however, the length scales and the physical origin of twisting in these systems is completely different (spontaneous phase chirality of b-sheets in amyloids and preparation history in extracted fibrils). There is also a very interesting and unique report of helical twisting obtained in electrospinning,[16] but the authors achieved it by clever engineering of their electric fields–as opposed to the spontaneous natural twist of the cellulose fibers reported here. We have applied three different imaging techniques to quantify this helical morphology: atomic force microscopy (AFM), scanning electron microscopy (SEM), and polarized reffection optical microscopy (POM). As a result of this characterization, we determined that the pitch-to-diameter ratio of helical fibers was about 7. Importantly, we also found significant proportions of fibers wound with either left-or right-handed helicity, which rules out a direct relationship with the chirality of cellulose and the underlying cholesteric mesophase. The results are interpreted with the help of a simple theoretical model for the tertiary structure of a chiral semiffexible filament, which is shown to wind into a helix along the axis of an external electric field.[17] The critical concentration for acetoxypropylcellulose (APC) is around 50% w/w, for which an isotropic phase coexists with an anisotropic (nematic) phase. At the isotropic-to-nematic transition the shear viscosity drops; this allowed us to electrospin a solution of concentration 60% w/w, hence in the nematic phase, whose viscosity is similar to that of diluted isotropic solutions (around 10 Pa s; the shear rate was 10 sÀ1). Figure 1 shows a typical micrograph of an APC fiber mat, at low fiber content to allow clear resolution of individual filaments. A large number of fibers are visible, which …