A superhigh-molar-mass polyelectrolyte, poly(methacryloylethyl trimethylammonium methyl sulfate) (PMETMMS), with molar mass 26 x 10(6) has been studied by viscosity and light scattering in saltless water-acetone mixtures of various compositions. PMETMMS is known to collapse on addition of acetone into an aqueous solution. It has been shown that the collapse of the polyelectrolyte chains may be induced also by dilution of the polymer solutions, leaving the solvent composition constant. The scattered Light intensity measured as a function of polymer concentration shows a maximum at a certain concentration. The position of the maximum is dependent on the solvent composition. A compact structure with the radius of gyration R-g approximate to 50 nm and the hydrodynamic radius R-h approximate to 30-40 nm is formed in a very dilute solution where the value of the second virial coefficient A(2) is close to zero. The values of M-w measured by static light scattering in solutions with various solvent compositions were constant. Correlation functions measured using dynamic light scattering unambiguously show a decrease of the relaxation times as well as the narrowing of the size distribution of the polymer with dilution. This most probably is due to the change of the balance of inter- and intramolecular interactions upon dilution.