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A technique is presented for accurately measuring the dispersion in single-mode fibres about a kilometre in length. It uses a polarisation interferometer and is simpler and less expensive than techniques currently being used. The interferometer incorporates a phase modulator and a test fibre. The dispersion is determined from the interference fringe seen when a sweep RF signal is applied to the modulator. The equipment is less bulky than that currently used for testing, so it can easily be carried into the field. Chromatic dispersion was measured with ps resolution in a 1 km-long single-mode fibre at wavelengths from 1.50-1.58 μm.