In-service fault location using optical time domain reflectometry is demonstrated for the first time over an installed submarine, fibre-optic cable. Results are presented showing simultaneous fault location and 565 Mbit/s data transmission on one fibre of the UK-Channel Islands no. 7 fibre-optic cable. In addition, laboratory experiments indicate that the technique could prove to be invaluable in supervising wavelength-routed networks incorporating both passive and active optical components. © 1990, The Institution of Electrical Engineers. All rights reserved.