We investigate the flow of a Casson fluid in contact with a Newtonian fluid in a circular pipe with permeable wall. The motion is caused by the movement of peristaltic waves on the flexible walls of the channel. The equation of the interface is obtained and its variation with the yield stress is discussed. The effects of yield stress and permeability on the pumping characteristics are studied in detail. Moreover, when the limit of the permeability parameter α and the yield stress 0 0, our results agree with those of Rao and Usha 25 for the peristaltic transport of two immiscible Newtonian fluids in a circular tube with an impermeable flexible wall. Furthermore, the results obtained for the flow characteristics reveal many interesting behaviors that warrant further study of the peristaltic transport models with physiological fluids, especially the shear-thinning fluids. Shear thinning reduces the wall shear stress. © 2009 Begell House, Inc.