The inclusion behavior between molecular nanotubes and linear polymer chains in aqueous solutions is experimentally investigated by measuring the optical absorption spectra of the solutions with iodine as a probe. It is found that the amount of polymer chains included into nanotubes increases with polymer length, which indicates a molecular recognition of the nanotube for the polymer length in the complex formation. Furthermore, the experimental results are compared with a theory based on the Flory-Huggins lattice model. They are in good agreement with each other and the inclusion energy between the molecular nanotube and the linear polymer chain is evaluated. (C) 2000 American Institute of Physics. [S0021-9606(00)70309-6].