Semiempirical direct dynamics simulations are used to study the potential role of CO in the gas-phase catalytic growth of carbon nanotubes. Calculations employing a fully self-consistent AM1 semiempirical electronic Hamiltonian predict that CO molecules can chemically adsorb to the open edge of partially grown zigzag and armchair nanotubes. The adsorbed CO molecules can form either pentagonal or hexagonal carbon rings by an electrocyclic reaction with neighboring adsorbed CO groups. Formation of hexagonal carbon rings in zigzag nanotubes is kinetically and thermodynamically favored by a CO insertion reaction with CO-saturated ends. These results provide some computational evidence that gas-phase CO can adsorb and contribute to the intermediate and latter stages of carbon nanotube growth. This study assumes that growth proceeds by the addition of carbon to the nanotube edge held open by metal catalyst particles, as opposed to a root growth or capped end growth mechanism.