In this paper, effects of one of the fibre nonlinearities, four-wave mixing, in long distance coherent multichannel optical fibre transmission are studied. An optical frequency division multiplexed system with 9 erbium doped fibre amplifiers, transmitting over 550 km of fibre is used in the expreiment. The nonlinear interactions between the channels due to the four-wave mixing are clearly observed. Experimental results are well agreed with the theoretical analysis and calculations. Results show that for multichannel transmission at several Gbit/s with conventional non-return to zero data, transmission performance is significantly affected by the four-wave mixing.