The focus of the present work is an experimental study of the behaviour of semi-dilute, opaque fibre suspensions during fully-developed pressure-driven flow in a cylindrical pipe. We measure the instantaneous velocity profiles across the radius R of the pipe, using ultrasound doppler velocimetry (UDV), as a function of the applied hydraulic pressure and concentration (0.75-1.75% (wt/wt)). In total 374 conditions were tested on three different flexible, non-Brownian fibre suspensions. The goal of the work was to gain insight into the role of the plug during transition to turbulence. From the UDV measurements, we estimated the radius of the plug r(p), the yield stress of the suspension tau(y), through knowledge of the pressure drop, as well as the Reynolds stress (rho u'(2)) over bar. We find that the yield stress varied non-monotonically with flow rate for each suspension tested. At slow flow, i.e. when r(p)/R -> 1, we observe that plug densification, i.e. a contraction of the plug created by the growth of a lubricating film at the wall, caused the initial increase in yield stress. Yield stress was found to continue to increase with flowrate and its maximum was reached at 0.4 < r(p)/R < 0.7. With plug sizes smaller than r(p)/R < 0.4, the yield stress of the plug diminished with increasing flowrate through what we believe to be an erosion-type mechanism. We estimate the critical Reynolds number Re-c for the disappearance of the plug for all cases. (C) 2014 Elsevier B.V. All rights reserved.