A visible fibre ring oscillator is demonstrated which is based on four-photon parametric gain caused by modulational instability. The modulational instability gain is accomplished in the normal dispersion regime by synchronously pumping both axes of a birefringent fibre. The modulational instability parametric oscillator output intensity exhibits 3-4 THz oscillations, and up to three Stokes bands are generated in the spectrum, ranging over 400cm-1. © 1992, The Institution of Electrical Engineers. All rights reserved.