A mechanism for phase-matched generation of second-harmonic (SH) light in an optical fibre has been experimentally and theoretically studied. A weak non-phase-matched SH signal, created via the quadrupole polarisation, initiates the self-writing of an axially periodic pattern of colour centres which is postulated to lead to the growth of a χ (2) grating. The grating (at the phase-match period) gives rise to SH generation at efficiencies of 0.5%. The spectral bandwidth of this χ (2) grating is 0.24 nm.