A modification in the conventional arc-discharge method for synthesis of nanotubes is presented. By injecting pure nitrogen gas directly into the plasma we have greatly increased the amount of boron nitride nanotubes produced. Isolated nanotubes and bundles were characterized by TEM. The vast majority of the nanotubes were double-walled with outer diameter around 3 nm. The predominance of double-walled BN nanotubes is seen as a direct result of the distribution of the number of graphitic BN layers for the nanococoons, second major product of the synthesis. Detailed HRTEM examination of the ends of BN nanotubes indicates continuity between the graphitic BN layers that coat boron nanoparticles, that is nanococoon, and the nanotube. At the other end of the nanotubes a flat angular cap was observed. HRTEM images of nanotube ends give support to a root-based growth mechanism. (C) 2003 Kluwer Academic Publishers.