This study investigates the improvement of performance of anaerobic fermentation of cattle manure mixed with the water lettuce Pistia stratiotes, a macrophyte plant growing in the effluent of an anaerobic digestor. The experiments used a series of continuous fermentors, using mixtures of cattle manure and water lettuce in the following proportions : 12.5 %, 16.6 %, 25 % and 50 % of Pistia stratiotes. The best biogas yields were achieved with a proportion of 50 % of water lettuce in the mixture giving a biogas yield of 0.62 m3/(m3. d.) with a methane content of 76.8 % over a 15-days hydraulic retention time, at a constant temperature of 35° C. The kinetic study based on batch fermentation shows that the process is well represented by the Monod Model. These performances are better than those obtained in anaerobic digestion of cattle manure used alone.