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A theory is developed for the excess photon noise in the detected photocurrent of the multimode laser. Two simple cases are treated: in one, the modes are uncoupled and in the other, they are phase locked. Both cases can be realized when a gas laser is operated under saturation conditions well above threshold, where complicated mode interactions make a detailed analysis difficult. As predicted, the excess photon noise is zero dB, within experimental error, for the phase-locked case; and it is approximately 20 ± 3dB for a stable vibration isolated laser operating in a multimode, nonlocked condition. Precautions have been taken to eliminate extraneous noise due to power supply ripple and the like, and the highly nonideal contribution of the photomultiplier has been subtracted out.