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We develop an in-service dispersion monitoring technique based on the optical frequency modulation (FM) method and examine its applicability to wavelength division multiplexing (WDM) long-haul transmission systems. This method is insensitive to the group-delay drift induced by environmental perturbations and can measure the dispersion value with its sign (normal or anomalous) accurately only from the received signal light. These features are very attractive for the application in ultralong-haul transmission systems. We implement the in-service dispersion monitor in a 32 × 10.7 Gbit/s WDM transmission system. Accurate dispersion monitoring at a transmission distance of 5950 km is achieved without any bit error rate (BER) penalty. Also, we discuss the applicability of the optical FM method to higher bit-rate systems from the experimental result.