[en] The reactivity signal of the KNK-II-plant shows almost harmonic oscillations of δrho <= 0.5 c. Very sensitive correlation measurements, made during the regular plant operation with the normal plant instrumentation, revealed, that these oscillations are associated with individual fuel elements. Auxiliary measurements under various operational conditions and theoretical considerations show, that this phenomenon is probably caused by flow-induced mechanical vibration. Similar characteristics with respect to the frequencies have obviously not yet been observed for fuel element vibration during tests in out-of-core loops and in other reactors. Therefore efforts have been made in order to classify the flow-induced vibration and to identify the particular excitation mechanism. Most likely seems a flow-induced vibration of whole fuel elements by vortex shedding or jet switching. This model can explain all observations without exception. (orig.)