Zero-shear viscosities ro of dichloromethane solutions of a semiflexible polymer, poly(n-hexyl isocyanate) (PHIC), were measured over a wide concentration range covering from dilute to concentrated isotropic regimes. Eight different molecular weight PHIC samples used for viscometry had the Kuhn statistical segment numbers N ranging from 0.80 to 97. The solution viscosity behavior of PHIC with N less than or similar to 10 resembled that of rigid polymers [e.g., poly(gamma-benzyl L-glutamate), schizophyllan, and xanthan], while ro for PHIC solutions with larger N almost obeyed the following power law at high concentration ranges : eta(0) proportional to C(3.6)M(4)(c, polymer mass concentration; M, molecular weight;). The power law behavior of eta(0) is characteristic of flexible polymer solutions, although the exponents for the PHIC solutions are different from those of flexible polymer solutions. The viscosity results of PHI:C solutions were compared with the fuzzy cylinder model theory recently proposed. Agreements between theory and experiment were found for PHIC solutions with N less than or equal to 8, but not for PHIC solutions with N greater than or equal to 15. From this result, it was concluded that the validity of the fuzzy cylinder model theory is limited to polymer chains with N smaller than ca. 10. No suitable theory was found for PHIC solutions with N greater than or equal to 15.