CoNi nanofibers (56 nm length, 6.6 nm width) and nanospheres (24 nm diameter) were synthesized by reduction of ions in a liquid polyol. These powders were then characterized by means of electron microscopy, N₂ adsorption, energy dispersive X-ray spectroscopy, X-ray diffraction and magnetometry. Similar properties were found for nanofibers and nanospheres, with the exception of shape and the consequent differences in the magnetization of the compressed powders that arise from the strong effect of fiber orientation on the demagnetizing field. Finite element method simulation was used to check this last point. These powders were also employed for the preparation of magnetorheological (MR) suspensions and their steady-state and dynamic rheological properties were studied under application of magnetic fields of different intensities. Both kinds of rheological measurements showed that the MR effect was considerably strengthened when nanofibers were used as dispersed phase instead of nanospheres.