We have observed that the normal to the polarisation plane of a whistler precesses around the geomagnetic field. Magnetic field observations by the Freja spacecraft have been used. These results were made possible by the use of an analysis which estimates the instantaneous polarisation properties of waves. During one event, the instantaneous normal of the polarisation plane precesses around the direction antiparallel to the geomagnetic field with an average deviation of 6¿. In total, the normal precesses four times around this direction as the whistler frequency decreases from 2 to 0.4 kHz in about 0.2 s. Available theoretical understanding can not unambiguously explain the precession. We show that the precession of whistler wave vectors can be observed, and suggest to use this new observable parameter to put new constraints on the theories of whistler propagation.