A boundary-layer transition study was carried out in the throat region of the DeLaval nozzle of a supersonic wind tunnel. The study was motivated equally by the need to find thresholds for laminar boundary-layer flow in the tunnel walls when roughness is present and by the desire to simulate transition on roughened blunt bodies in supersonic and hypersonic flow. Detailed inviscid and viscous flow measurements were done from the low subsonic to the supersonic regions of the nozzle throat. The roughness was caused by attaching distributed roughness overlays on the nozzle surface. Transition, detected by hot-film anemometers, was found to move upstream as the flow Reynolds number and/or the roughness height increased. Cast in the coordinates of some of the empirical blunt-body transition correlations currently in use, the present transition data agree with the available blunt-body data when the nondimensional roughness exceeds unity and support the concept of a constant roughness Reynolds number for transition in that regime. At the lower roughness heights, the results show that the transition Reynolds number departs from the aforementioned correlations and approaches a limit insensitive to roughness but characteristic of the experimental facility.