To critically compare with the fuzzy-cylinder-model theory for the dynamics of stiff-chain polymers in solution, zero-shear viscosities and mutual diffusion coefficients have been measured for dilute through concentrated isotropic solutions of poly(n-hexyl isocyanate) (PHIC) samples with the number N of Kuhn＇s statistical segments between 0.21 and 55. The theory describes quantitatively both viscosity data for PHIC samples with N smaller than 20 and diffusion coefficient data for PHIC samples with N up to 26, with a single set of hydrodynamic parameters characteristic of PHIC. This confirms the validity of the theory to explain the global-motion dynamics of semiflexible polymer chains in solution. On the other hand, such a quantitative description is not achieved with zero-shear viscosities for PHIC samples with N larger than 20, where a power-law concentration dependence appears at higher concentrations in a form different from a scaling law seen for flexible polymer solutions. This indicates the limitation of the validity of the fuzzy-cylinder-model theory.