Sinusoidal length deformations were applied to bundles of one to three fibres from glycerol-extracted Lethocerus flight muscle under conditions of high calcium activation (pCa=6) and in the absence of phosphate. The mechanical response was found to be significantly non-linear under these conditions for amplitudes of the order of 0.05% and greater. The non-linearity was analysed into harmonic components by Fourier analysis, data for this being collected and stored in a computer. The largest harmonic components were found to be the first and second; these components were particularly prominent at frequencies close to those producing maximum phase shift between length and fundamental tension amplitude. At very low amplitudes (<0.02%) the non-linearities became very small and could not be readily detected. At this very low amplitude the fundamental could be considered as the linear response to the length change; it had a much sharper frequency dependence than previously recorded from Lethocerus flight muscle. The variation of the response with imposed conditions has been studied.