A simple relation is presented for the viscosity of isotropic solutions of flexible, semiflexible and rodlike chains over a range of concentration from dilute solutions to undiluted polymer, chain contour lengthLand persistence lengthâ. The expression is designed to account for the separate effects of screening of thermodynamic and hydrodynamic interactions, and the onset of intermolecular chain entanglements under Flory theta solvent conditions, or in ‘‘good’’ solvents, for which intramolecular excluded volume is important in dilute solutions. The relation includes a revised (dimensionless) Fox parameterX̃cthat determines the onset of intermolecular entanglements, whereX̃c=πNAρLcâ/3MLfor semiflexible polymers withâ≪L, with ρ the polymer density,MLthe mass per unit length of the chain, andLcthe value ofLfor the onset of entanglement effects in undiluted polymer;X̃c≊100 for a number of systems, including rodlike chains.