The adsorption of the biopolymer chitosan on the (5,0) and (5,5) boron nitride nanotubes (BNNTs) was theoretically analyzed by Molecular Simulations. The calculations were done using the well known density-functional theory within the local density approximation; the exchange-correlation term was modeled with the Perdew-Wang parameterization. The structural stability was based on the obtention of the minimum energy and non-complex vibrational frequencies. Different sites, charge (neutral, cation and anion) and orientations of chitosan, using the monomer unit, were considered. For the (5,5) nanotube, the minimum energy site corresponds to that when the monomer (for all charges assigned) is perpendicular to the nanotube; only structural stability was found in the anion assignment for the (5,0) nanotube. The chemisorption process is through the amine group modifying the semiconductor character of the nanotube to semimetallic one, with ionic properties, suggesting their solubilization on hydrophilic nanomaterials. (C) 2012 Elsevier B.V. All rights reserved.