Measurements of the toroidal magnetic field inside the JET tokamak plasma by means of the motional Stark effect are reported. In the vicinity of the resonance layer ion-cyclotron-resonance-heating (ICRH) generates a population of energetic ions. For the first time the diamagnetic change in the toroidal magnetic field due to these ICRH fast ions has been measured. A magnetic field decrease up to 4% has been observed. Oscillations of the toroidal magnetic field caused by sawtooth oscillations of the thermal plasma pressure have been resolved. First results of the local-increase slowing-down time of the fast particles are presented.