The spatial and orientational distributions of fibers in laminar and turbulent pipe flows are simulated numerically. The simulated results are consistent with the experimental data available in the literature. In the laminar flow regime, more fibers are aligned with the flow direction with increasing Reynolds number. The shear rate of fluid around a fiber is the key factor for determining the orientation distribution of fibers. The fibers aspect ratio and fiber density have insignificant influence, while the fiber velocity and the fiber Stokes number have marginal influence, on the orientation distribution of fibers. In the turbulent regime, the spatial and orientational distributions become more homogeneous with increasing Reynolds number, and the fluctuation intensity of fiber velocity in the streamwise direction is larger than that in the other two directions, in contrast to the fluctuation intensity of the fiber angular velocity, which is weaker in the streamwise direction. (C) 2003 Elsevier Ltd. All rights reserved.