A stabilizer controlling the current of a large electromagnet in a Q-band electron spin resonance spectrometer is described. The maximum field strength of 12500 G is produced when 13 A flow through the magnet coils. Under normal conditions, after an initial warming up period, a stability of ±0.25 G or ±2 parts in 105 is attained for mains voltage fluctuations of ±5%, measured during one hour.