The rheological behavior of model suspensions consisting of a Newtonian silicone oil and fibers of different flexibilities have been investigated in steady and transient shear flows. Various fiber suspensions have been prepared to examine the effect of flexibility parameters (stiffness and aspect ratio) as well as the role of interactions in the semi-dilute and semi-concentrated regimes. The viscous and elastic properties of the fiber suspensions are shown to be strongly enhanced by fiber flexibility. With increasing flexibility, the suspensions also exhibit enhanced shear-thinning and the shear rate for the onset of shear-thinning decreases. In start-up flow and especially at low shear rate, the suspensions show large stress overshoots, and under reversal flow delayed overshoots. The magnitude of the overshoot increases as fiber flexibility gets larger. This last effect has been attributed to less orientation in the flow direction during the forward flow for the more flexible fibers. Finally, the effects are more pronounced in the semi-concentrated regime, and this is attributed to enhanced fiber-fiber interactions. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3103546]