A method for operating a magnetic-inductive flow meter in which the presence of a parasitic magnetic field is detected energizing of electromagnets with at least one first current value and at least one second current value. The measuring apparatus determines first and second measured values from energizing of the electromagnets with the first and second measured values and a first pair of measured values is formed from the first current value and the first measured value and a second pair of measured values is formed from the second current value and the second measured value, the correspondence of the pairs of measured values and of the function of permeability is determined. If correspondence of the pairs of measured values and of the function of permeability is not found that fact is signaled, the lack of correspondence being due to the at least partial presence of a parasitic magnetic field.