An apparatus (104) mitigates cross-channel nonlinear distortion of an optical signal (138) carried on one of a plurality of wavelength channels (118) in a wavelength division multiplexed (WDM) transmission system (100). The apparatus includes a first optical receiver (126) which is arranged to detect a measure (134) of aggregate optical power of the plurality of wavelength channels. A nonlinear dispersion compensator includes means (144) for applying a phase modulation to the optical signal in proportion to the measure of aggregate optical power.