The flow and transition of fiber suspensions to turbulence in a rotary shear tester were studied. Both torque and LDA measurements were performed on a refractive-index-matched glass-fiber suspension in the 3–20 wt. % concentration range. The flow of such suspensions underwent two transitions: the first one coincided with complete agitation, a sharp increase in torque, and fluctuating velocities close to zero. The mean velocities were similar to those of single-phase flow above this transition. The fluctuation velocities approached those of single-phase flow with increasing rotational speed, until they were nearly equal. This second transition was interpreted as fully developed turbulent flow. CFD, assuming single-phase flow, is a helpful tool for describing the flow behavior in this region. The network strength was useful for the study of flow transitions. LDA measurements of the boundary layer at the bottom wall showed that the velocity profile is linear close to the wall even with fibers present. The presence of the fibers also flattened the profiles, indicating an increased momentum transfer.