High-resolution VHF Doppler radar measurements during a jet stream passage reveal radial velocity oscillations produced by a Kelvin-Helmholtz (KH) instability. A detailed investigation of the velocity data, using digital filters and least squares methods, shows that the growth rate of the KH oscillations decreases linearly with increasing time so that a growth and a decay stage can be defined. During the occurrence of the KHI, the radar signal power is characterized by periodic bursts up to 20 dB above the mean power level. The bursts have a period equal to that of the KH oscillation and are restricted to heights around the critical level at which the KH instability is generated. Model computations indicate that the power bursts are produced by static instabilities which are due to KH-induced superadiabatic lapse rates. During the growth stage, the vertical profile of the mean wind exhibits characteristic temporal changes that can probably be ascribed to the Reynolds stress of the KH instability. There is some evidence that the observed KH oscillations are parts of a sequence of KH waves initiated by long-period gravity waves.