Measurements of viscoelastic properties in good solvent semidilute solution are reported. for three linear polymer species : polystyrene, polyisoprene, and polybutadiene. It is shown that using the same reduced concentration C/C* as for static properties, three different curves are obtained for the reduced viscosity eta/eta(solvent). The scaling used for static properties is no longer valid and a number n(e) of blobs per chains are needed for polymers to be entangled. This number, here determined by the ratio of the osmotic modulus to the elastic modulus K/G = n(e), is found to be different for the three kinds of polymer and identical to the number of monomers per entanglement measured in the melt. The reduced variables for the viscosity are eta/eta(Rouse) as a function of C/C-e where eta(Rouse) = eta(solvent)(C/C*)(1.30) is the viscosity predicted by the Rouse model and C-e = n(e)(0.76)C* is the concentration at which polymers begin to be entangled.