The structural order of short nucleic acid fragments (having a mean length of 170 nm) in aqueous solution in the presence of high magnetic fields (up to 18.5 tesla) has been investigated by small-angle neutron scattering, light diffraction and by precision measurements of the magnetic birefringence. Our data give clear evidence that, above a critical concentration, the semi-rigid electrically charged fragments arrange themselves into a periodic lattice having an interparticle spacing of ∼6 nm. Neighbouring rods show a nearly parallel orientation, but a slight twist seems to exist, leading to a well defined pitch of the order of 1000 nm, which gives rise to a strong diffraction of visible light. The unexpectedly low saturation of the birefringence in the high magnetic field, however, indicates that the order is not of the simple cholesteric type. The forces which are responsible for inducing the twist across the large interparticle distance are mainly anisotropic Van der Waals forces.