Ultrasonic Doppler velocimetry (UDV), a non-invasive fluid flow measurement technique suitable for use in opaque systems, was used to measure pulp suspension flow velocity in a rectangular 1 : 11-scale agitated pulp stock chest. The flow characteristics of a Maxflo impeller were measured as a function of fibre mass concentration, fibre type and impeller speed. Using the velocity profiles measured, the impeller pumping rate and flow number were calculated as a function of suspension yield stress (itself a function of fibre mass concentration and fibre type). The flow number was found to decrease as the suspension yield strength increased. For example, when the mass concentration of a hardwood pulp was increased from 0% (water) to 3.3%, the flow number decreased from 0.64 to 0.43. UDV was also used to identify regions of poor mixing by measuring flow profiles at critical locations within the vessel. This study also demonstrates the ability of UDV to measure the cavern size of the pulp suspension during agitation.