The production of boron nitride nanotubes has prompted numerous investigations into the structural properties of intermediate-sized boron nitride molecules. For molecules with more than twenty atoms, cage isomers are the most stable. In this study, two classes of boron nitrides are compared: fullerene-like structures consisting of pentagons and hexagons, and alternant structures consisting of squares and hexagons. These two classes are compared for B13N13, B14N14, and B16N16 by theoretical calculations using Hartree-Fock theory and density functional theory (B3LYP and LDA). The major result is that the alternant structures are more stable than the molecules based on fullerenes. (C) 2003 Published by Elsevier B.V.