An experimental study was carried out to elu- cidate the large-scale vortical structure in a separated and reattaching flow over a backward-facing step. The Rey- nolds number based on the step height (H) was ReH=33,000. The large-scale vortical structure was probed by means of three-dimensional velocity measurements performed at the recirculation zone (x/H=4.0) and the reattachment zone (x/H=7.5). A 32-channel microphone array extending in the streamwise and spanwise directions was used for sensing the wall pressure fluctuations. The relationship between the flow field and the relevant spatial mode of the pressure field was determined by examining the spatial box filtering. From the relevant spatial mode of the wall pressure fluctuations, a conditional averaging technique was employed to characterize the coherent structure. In addition, the cross-correlation between velocity and wall pressure fluctuations was calculated to identify the structure and the length scale of the large-scale vortex. The cross-correlation results revealed that the large-scale hairpin vortices have a three-dimensional structure, in agreement with previous findings. The pres- ent results clearly show the growth and downstream elongation of the hairpin vortices. List of symbols H step height, m k turbulent kinetic energy, m 2 /s 2 q freestream dynamic pressure, Pa ReH Reynolds number based on U0 and H,UoH/m U0 freestream velocity, m/s Uc convection velocity, m/s X0 streamwise coordinate of the measurement origin, m xR time mean reattachment length, m