Measurements were made on a 4-in.-od hollow-cylinder model with its center line adjusted parallel to the flow in the NOL 40 x 40-cm Aeroballistics Intermittent Wind Tunnel no. 2 at 6 Mach numbers between 2.2 and 5.0. At these Mach numbers, supersonic flow was attained inside the cylinder. Pressure distributions on the hollow cylinder were nearly the same as the free-stream static pressure distribution. The location of boundary-layer transition was determined from schlieren photographs. The transition Reynolds numbers for the hollow cylinder decreased with increasing Mach number. The results of the boundary-layer transition were compared with those obtained previously on a 5 deg cone and a 20 deg cone-cylinder in the same tunnel.