Triangular-profile monomode fibre having minimum dispersion at 1.55 μm has been made with an attenuation of 0.24 dB/km at this wavelength. This has better optical performance than fibres optimised for 1.3 μm operation which have typically 1.5 to 2 times this loss at their wavelength of minimum dispersion. The spot sizes of the two fibre types are similar at their respective optimum wavelengths.