Intrachannel four-wave mixing (IFWM) represents main source of impairments in fiber transmission at 40 Gb/s. A number of phase modulation techniques have been proposed to suppress the IFWM. In this paper, we study a cost-effective way of increasing the nonlinear tolerance of a 40-Gb/s wavelength division multiplexing transmission system by asynchronous phase modulation (APM). This can be achieved with one phase modulator placed after the wavelength multiplexer so that cost is shared by all channels. We show, by means of numerical simulations and laboratory experiments, that APM greatly improves the performance of ON-OFF-keying transmission.