A transmission experiment was performed to study the high-speed potential of a long-distance gigabit lightwave system employing a single-frequency 1.5 μm laser and a new long wavelength III–V avalanche photodetector. A 120 km length of standard production single mode fibre was used in the test, in which a bit error rate of 2×10−10 was achieved at 1 Gbit/s. This distance is the longest reported to date for rates higher than 500 Mbit/s, and the product of bit rate and distance (120 km Gbit/s) is the highest value achieved for any bit rate. The distance was limited by the 32.3 dB loss of the transmission path together with a total of 5.6 dB in power penalties associated with chirp broadening of the laser line.