The number of colour centres generated by gamma radiation has been measured with the ESR method; radiation-induced loss was obtained during the steady-state irrediation. By comparing these data, the induced loss in Ge/P-doped GI fibres corresponding to one colour centre is estimated to be 5.5 × 10−15 dB/km. The number of colour centres generated by gamma radiation has been measured with the ESR method; radiation-induced loss was obtained during the steady-state irrediation. By comparing these data, the induced loss in Ge/P-doped GI fibres corresponding to one colour centre is estimated to be 5.5 × 10−15 dB/km.