An internal bias generator for providing a negative bias voltage to the substrate of an MOS integrated circuit at a magnitude higher than the power supply voltage includes a pump circuit which comprises a plurality of switches which are sequentially actuated by nonoverlapping clock signals to alternately charge and discharge a capacitor. The clock signals are produced by a generator which includes a series of RC-delay inverting amplifier stages coupled to a series of NOR gates. The bias generator further comprises a threshold-sensitive regulator which uses the source-body effect of substrate bias on the threshold voltage of an MOS FET to control the magnitude of the applied bias voltage. When the sensed threshold voltage deviates from a desired level, certain of the clock signals are disabled, thereby to modify the bias voltage applied to the substrate in a manner to tend to restore the threshold voltage to its desired level.