Flow visualization experiments are conducted to study the phenomena of separation and reattachment behind a segmented step in subsonic turbulent flow. The results show that the effect of castellations on the step face is to produce strong three‐dimensional disturbances in the initial recirculation region. This is followed by a complex reattachment process and a transition region in which the three‐dimensional disturbances are attenuated, the flow eventually resembling a two‐dimensional flow in the relaxation far downstream.