The interfacial tension γ between the coexisting isotropic and nematic phases of toluene solutions of poly(n-hexyl isocyanate) (PHIC) was determined as a function of the molecular weight of PHIC by the pendant drop method. The value of γ was almost independent of the molecular weight, in contrast to the theoretical prediction for rodlike polymer solutions that γ should be inversely proportional to the polymer molecular weight. This is consistent with the disagreement between rod theories and experiment with respect to the isotropic-nematic binodal concentrations of PHIC solutions and implies the importance of the chain-flexibility effect on γ in the solutions.