The rheo-optical response to shearing of semidilute solutions of highly monodisperse, wormlike poly(n-hexyl isocyanate) three-arm stars was investigated using simultaneous birefringence and dichroism measurements. There were two modes to the birefringence response : a fast mode, corresponding to molecular orientation and a slow mode which, through scattering dichroism measurements, is confirmed to be associated with the dynamics of anisotropic multichain associations in solution. In addition, the dichroism measurements showed that these structures oriented perpendicular tot he mean chain orientation within the structures. The optical equivalent of the shear stress, n(xy), scales as ((y) over dot c(3.2))(0.74) for these stiff, low molecular weight samples in the semidilute concentration.