An instrument for the nondestructive measurement of attenuation in fibres for optical communications and for checking the system laser and detector in situ is described. The instrument is the first of a range of instruments designed to cover all test requirements on optical fibres and incorporates its own gallium aluminium arsenide laser source. The attenuation of fibres with a range of core sizes, and whose losses may be as much as 60 d may be measured to within ±1 dB accuracy. The method involves comparison with a short (zero dB loss) piece of similar fibre. The system detector can be checked for golno go performance and the instrument's detector can be calibrated to read the mean output power of the system laser.