Pseudopolyrotaxane 2 is synthesized from polyviologen (1) and cucurbituril (CB[61) in water by simple stirring at room temperature. The degree of threading (number of CB[6] beads per repeat unit) can be precisely controlled from 0.1 to 1.0 by controlling the amount of CB[6] added. In 2, CB[61 beads are localized at the middle of the decamethylene units of the polymer through hydrophobic and charge-dipole interactions, to afford a well-defined microstructure in aqueous medium as found by NMR studies. The pseudopolyrotaxane has higher intrinsic viscosity, but a lower shape-dependent Huggins constant, than the parent polymer. This is attributed to a larger hydrodynamic radius and more expanded chain conformation of the pseudopolyrotaxane compared to those of the parent polymer, The decomposition temperature of 2 increases with increasing amount of CB[6] threaded on the polymer.