We have made what we believe is the first experimental observation of the switching transients resulting from single flux quantum transitions in superconducting devices. For that purpose, we used ``long'' Josephson junctions or interferometers which exhibit overlapping vortex modes. By applying a magnetic field to such devices, vortex‐mode transitions are induced. The resulting transients are detected by a small Josephson junction located nearby. The experimental results show unambiguously that the points where a signal is detected by the sense junction correspond to the vortex‐mode boundaries of the device. These results have important implications for memory applications.