We investigate the origin of several paramagnetic defects in the boron nitride nanotubes and nanofibers prepared by different methods and compare them with conventional BN powders. All BN samples contain an intrinsic paramagnetic defect with g = 2.003. This defect is assigned to the electron trapped at the nitrogen vacancy. In addition to the g = 2.003 defect, BN nanotubes contain substantial amount of Mn and Cr impurities. We found that these impurities are not incorporated into the BN lattice but originate from the MgO nanoparticles trapped inside the BN nanotubes. (c) 2005 Elsevier B.V. All rights reserved.