Acoustophoretic (AP) motion of spherical polystyrene particles in a steady pressure driven flow is investigated in shallow microchannels, where the channel height is comparable to the particle diameter. Particle trajectories at different ultrasonic actuation amplitudes are extracted by a particle tracking algorithm. Depths of the particles are predicted using the streamwise particle speed that is due to the pressure driven flow. The particle depths are shown to be influenced by the actuation voltage. The particle migration along the channel height is explained using the second-order perturbation theory. The particle equation of motion is employed to extract the AP force. Wall effects are included in the analysis of both particle depth and force predictions. Differences as large as 20% in the AP force magnitude due to the wall corrections are reported. The AP force is also calculated using the theoretical force expression, and compared with the experimental results. The focal length, which is the necessary distance to effectively concentrate particles in a microchannel, is calculated using the analytical solution of the particle equation of motion. The calculated force and the focusing length agree well with the experimental results. The focal length is critical for the design of micro sample concentration devices. (C) 2010 Elsevier Inc. All rights reserved.