Light and small-angle X-ray scattering and viscosity measurements in tetrahydrofuran at 25 degrees C were made for nine helical poly[5,8-dimethyl-6,7-bis-(propoxymethyl)quinoxaline-2,3-diyl] samples ranging in the weight-average molar mass M-w from 8 X 10(3) g mol(-1) to 6 X 10(5) g mol(-1) to determine the particle scattering function, the radius of gyration, and the intrinsic viscosity as a function of M-w. The dimensional and hydrodynamic properties were consistently explained in terms of the Kratky-Porod wormlike chain. The helix pitch per residue h (or the contour length per residue) and the chain stiffness parameter lambda(-1) (the Kuhn segment length or twice of the persistence length) were estimated to be h = 0.19 nm and lambda(-1) = 43 +/- 3 run. The former parameter corresponds to an internal rotation angle of about 1200 which is substantially the same as the most stable helical structure estimated from the internal rotation potential. The latter one (lambda(-1)) indicates that the poly(quinoxaline-2,3-diyl) has a rigid helical main chain of which the dihedral angle between adjacent monomer units are restricted by steric hindrance.