During April 11–13, 1978, a jet stream passed over Germany. A synopsis of this event, deduced from radiosonde data taken every 6 hours at five German weather stations, is given. During the maximum overhead passage the jet stream has been observed continuously using the SOUSY (sounding system) VHF radar facility. With an eastward pointing antenna beam, one component of the horizontal velocity was derived. Height profiles of the eastward jet stream velocity for a period of 11 hours are presented for the altitude range 3.3–10.65 km with a spatial resolution of about 150 m. These profiles show a pronounced maximum at the jet stream altitude which agrees well with measurements of the nearest radiosonde ascent. A shear instability generated by the strong vertical gradients of the horizontal velocity at the bottom of the jet stream is observed. The results clearly indicate that the spatial scales of such processes are of the order of 100 m. A height resolution of 150 m, as is used here, is necessary to resolve these processes.