The viscoelastic behaviour of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer in aqueous solution forming a face-centred cubic (fcc) micellar phase has been investigated using oscillatory shear rheometry. With increasing strain amplitude, the micellar solution was observed to undergo a transition from linear to non-linear behaviour, characterized by strong shear thinning. The non-linear behaviour observed in the stress response was analyzed by Fourier transformation of the waveform. Fourier analysis revealed that the high harmonic contributions to the shear stress response increased with strain amplitude and up to the 81st harmonic was observed for very large amplitudes. The onset of non-linear response as defined from the dependence of isochronal dynamic shear moduli on strain amplitude was found to be in good agreement with that defined by the appearance of a higher harmonic in the stress waveform. The amplitudes of the harmonic coefficients are compared to the predictions of a model for the nonlinear rheological response of a lyotropic cubic mesophase based on the stress response to a periodic lattice potential (Jones and McLeish 1995). It is found that the model is able to account for qualitative trends in the data such as the development of finite higher harmonics with increasing strain, but it does not describe the full frequency and strain dependence of these coefficients.