An optical-fibre wavelength-selective directional coupler has been fabricated using evanescent-wave coupling between fibres of dissimilar core radii and refractive-index profiles. This device has a nonperiodic filter response with a bandpass of 90 nm FWHM centred around 1.22 μm, and has a peak coupling efficiency of 60% with an excess loss of 0.2 dB.