Method for controlling a group of glow plugs in a diesel engine, which are connected with a direct current source via individual supply lines and which are to be controlled by a pulse width modulation process at the same temperature, at least in time average. The electric resistance of the glow plugs, less the resistance of the supply line to the heating element of the glow plugs, is determined during operation of the engine and a relative pulse width at which the glow plugs are to be operated is calculated from the value so obtained.