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. © 1974 American Institute of Physics.