The use of directly modulated DFB lasers for 8/spl times/2.5 Gbit/s WDM systems has been demonstrated over a 6000 km straight line test-bed. The influence of the transmitter spectral width has been investigated on a 8/spl times/2.5 Gbit/s WDM 6000 km straight-line test-bed. The measurements show that, even with a 10 GHz polarisation scrambling induced spectral broadening, the penalty is kept below 1 dB. In agreement with these results, the transmission of a directly modulated DFB laser has been successfully demonstrated.