A magnetic-inductive flowmeter having a measuring tube for guiding a conductive medium, having a magnetic field generator for generating a magnetic field passing through the measuring tube at least partially perpendicular to the medium flow direction, electrodes for tapping a measuring voltage induced in the medium and a conductivity measuring device for determining the conductivity of the medium. The conductivity measuring device has a measuring circuit with the electrodes, a voltage source and a measuring unit. In operation, the voltage source generates a voltage and the measuring unit measures the actual electrode voltage applied between the electrodes. The control unit sets the actual electrode voltage to a predetermined constant setpoint value and the evaluation unit determines the electrode current flowing between the electrodes and calculates the conductivity value of the medium or a variable related to the conductivity using the determined electrode current and the actual electrode voltage.