The first cumulant Omega(k) of the dynamic structure factor as a function of the magnitude k of the scattering vector was determined from dynamic light scattering measurements for three poly(n-hexyl isocyanate) (PHIC) samples with weight-average molecular weights equal to 1.04 x 10(5), 3.63 x 10(5), and 7.71 x 10(5) in n-hexane at 25.0 degrees C. The mean-square radius of gyration [S-2], the translational diffusion coefficient, and the intrinsic viscosity were also determined from static and dynamic light scattering and viscosity measurements, respectively, to characterize the samples, it is shown that the present data for PHIC as a typical semiflexible polymer along with the previous data for atactic polystyrene and the literature data for DNA are consistent with the theoretical prediction on the basis of the helical wormlike chain model for the dimensionless quantity eta(0)Omega(k)/k(B)Th(3) as a function of the reduced magnitude [S-2](1/2)k Of the scattering vector, where eta(0) is the solvent viscosity, k(B) the Boltzmann constant, and T the absolute temperature. In general, this dimensionless quantity appreciably depends on the kind of polymer even for flexible polymers.