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Digital systems using multimode fibres with a step-index profile, a lossy cladding and a low-loss core are treated in the letter. The influence of fibre numerical aperture (n.a.) on repeater spacing is examined when the total core-cladding interface loss results from Fresnel reflection. It is shown, by way of example, that in this case there is no advantage in using low n.a. values, even at moderate modulation rates, owing to a diminishing receiver incident optical power with decreasing n.a.