Light scattering and viscosity measurements were made on dilute solutions of a polymacromonomer consisting of a polystyrene main chain and polyisoprene (PI) side chains, which was synthesized by anionic polymerization. The second virial coefficient A(2) in 1,4-dioxane was positive at the theta temperature for linear PI, 35.3 degrees C, and became zero at 31.0 degrees C. The positive A(2) at 35.3 degrees C was ascribed to the effect of the intermolecular interaction between the PI unit and the main-chain unit. From analyses of the mean-square radius of gyration, intrinsic viscosity, and A(2) obtained as functions of molecular weight, the stiffness parameter lambda(-1) and the excluded-volume parameter B in cyclohexane at 25.0 degrees C (good solvent) and in 1,4-dioxane at 35.3 degrees C were determined. The experimental lambda(-1) for cyclohexane solutions was close to the value predicted by the first-order perturbation theory, but the value for 1,4-dioxane solution was larger than theoretical one, showing that the contribution of the interaction between the side-chain and the main-chain units is essential. The smoothed-density theory for B gave a close value to the experimental one for cyclohexane solutions, while it gave a much larger value than the experimental one for 1,4-dioxane solutions. This was attributed to the defect of the theory ignoring the chain connectivity.