A 5B6B line-coded 565 Mbit/s system designed for single-mode transmission at 1.3 μm wavelength was successfully operated through 37 km of state-of-the-art graded-index fibre (50/125 μm) made by PCVD process. With mean bandwidth values in excess of 1.5 GHz for the individual 2.18 km-long cable sections and a low concatenation factor (γ≃0.54), an overall fibre bandwidth of 400 MHz was achieved. The measured receiver sensitivity was −32.5 dBm for a 10−10 BER, showing a penalty of ≤3 dB compared to single-mode transmission.