The rheo-optical response of semidilute solutions of monodisperse, wormlike poly(n-hexyl isocyanate) (PHIC) in simple shear flow was investigated using a flow birefringence technique. The optical equivalent of viscosity eta(b) as measured by birefringence was found to depend on the contour length L, concentration c, and the ratio of contour length to persistence length L/alpha. The power exponent of the concentration dependence of viscosity increased with increasing L/alpha with eta(b) similar to C-3.2 Ibr L/alpha congruent to 0.8, eta(b) similar to c(4.0) for L/alpha congruent to 1.8, and eta(b) similar to c(4.2) for L/alpha congruent to 3.3. A simple scaling relation for viscosity, eta similar to c(4) (L-3Le8), where L-e is the effective rigid rod length of the wormlike chain and is a function of L/alpha, is derived for the regime L congruent to a and substantiated experimentally. Further, PHIC solutions at concentrations below 6% exhibited two well separated relaxation modes after flow cessation. While the fast mode is attributed to molecular reorientation, dynamic light scattering confirms that the slow mode may be attributed to the formation of large multichain structures.