A model is presented to describe the orientation and concentration state of semi-dilute, rigid ﬁber suspensions in a rectangular channel ﬂow. A probability distribution function is used to describe the local orientation and concentration state of the suspension and evolves according to a Fokker-Plank type equation. Long range hydrodynamic ﬁber-ﬁber interactions are modeled using the approach outlined by Folgar and Tucker (J. Reinforced Plast. Comp. 3 98–119 1984). Near the channel walls, we apply the no-ﬂux boundary conditions proposed by Schiek and Shaqfeh (J. Fluid Mech. 296, 271–324, 1995). Geometric constraints are used to couple the ﬁbers’ rotary motion with its translational motion. This eliminates physically unrealistic orientation states in the near-wall region. A two-way coupling between the ﬁber orientation state and the momentum equations of the suspending ﬂuid is considered. Experiments are performed to validate the numerical model by visualizing the motion of tracer ﬁbers in an index-of-refraction matched suspension. The orientation distribution function is determined experimentally as a function of channel height. The results indicate that at distances less than one half ﬁber length from the channel walls, the model accurately predicts the available ﬁber orientation states and the distribution of ﬁbers amongst these states. The model further predicts a sharp concentration gradient in this region.