Optical transmission at 20 Gbit/s over standard singlemode and dispersion-shifted fibre has been investigated. By exploiting the chirp characteristic of the LiNbO3 modulator, a sensitivity penalty of less than 1 dB has been demonstrated over 20 km of standard fibre. With dispersion compensation, transmission over a single 80 km span of standard fibre was achieved with –32.7 dBm sensitivity. Transmission over 420 km of dispersion-shifted fibre was also demonstrated with an optical sensitivity of –31.1 dBm.