Stability features are studied experimentally for the unstable three dimensional boundary layer flow on a swept‐back flat plate. A pressure gradient on the flat plate is induced by a displacement body. Infinite sweep conditions are approximated by means of contoured endplates. For the measurements, hot‐wire and surface hot‐film anemometry as well as flow visualization techniques are used. In addition to stationary waves, traveling waves are also traced. The cross‐flow Reynolds numbers for the first appearance of either instability mode are of approximately the same magnitude. Wavelength and the direction of stationary vortices, as well as the frequencies of the most amplified traveling waves, are measured for different Reynolds numbers. The data obtained by the measurements are compared with the results of linear stability theory. The location of the final transition on the swept flat plate has proved to be fairly well predicted by the empirical transition criterion of Coustols (The`se de Docteur Inge´nieur, Ecole Nationale Supe´rieur de l’Ae´ronautique et de l’Espace, Toulouse, 1983).