Dilute through semidilute solutions of a semiflexible polymer, poly(n-hexylisocyanate) (PHIC), dissolved in a good solvent, dichloromethane (DCM), were studied by static light scattering as well as by sedimentation equilibrium. The solutions showed the following characteristic features, which were distinct from those of flexible polymer-good solvent systems. (1) The second virial coefficient was independent of the molecular weight. (2) The reduced third virial coefficient was much smaller than that for a flexible polymer in a good solvent. (3) The concentration dependences of the osmotic compressibility (partial derivative c/partial derivative II), and correlation length xi did not obey the scaling law nor the prediction of the renormalization group theory that was successful for flexible polymer-good solvent systems. Except for the correlation length results, the above features were favorably compared with a scaled particle theory including a weak attractive interaction. The present study is an advance on a previous work (Sate, T.; Teramoto, A. Mol. Cryst. Liq. Cryst. 1990, 178, 143) in that thermodynamic properties of semiflexible polymer solutions are understood on a more realistic molecular basis taking an attractive potential into account.