Error-rate measurements made on a 1 Gbit/s fibre-optic communications link are described. The link utilises a GaAlAs d.h. injection laser, a 1.6 km single-mode fibre, an avalanche photo-diode and a multiplexer and demultiplexer for four 250 Mbit/s channels. A comparison of the measured results with calculated data using a Gaussian approximation showed good agreement within 2 dB.