Measurements of the toroidal magnetic field inside the JET tokamak plasma by means of themotional 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 dueto these ICRH fast ions in combination with a pressure increase of the thermal population has been measured. Amagnetic field decrease of up to 4% has been observed. Oscillations of the toroidal magnetic field caused by sawtoothoscillations of the thermal plasma pressure have been resolved. First results of the local β increase due to ICRH andthe slowing-down time of the fast particles are presented.