The incompressible turbulent flow over a backward-facing step in a rectangular duct was investigated. The main focus was on the effects of the side walls on the centerplane characteristics of the flow. The Reynolds number based on the step height and oncoming freestream velocity was 26,200 and aspect ratios (step span-to-step height) ranging from 2 to 28 were investigated. For this range of aspect rations, the pressure distributions along the centerline of the step and flat walls were obtained. In addition, streamwise and transverse centerplane velocity distributions were obtained for aspect ratios 2, 4,8,12 and 20 using an x-type, hot-wire probe.