Abstract: Three experiments were conducted to investigate the effects of disodium fumarate on CH₄ production and fermentation by rumen micro-organisms. In experiment 1, rumen contents were collected from three local goats. Disodium fumarate was added to the incubation bottles to achieve final concentrations of 0, 4 and 8 mmol/L-fumarate. Results showed that disodium fumarate addition increased the final pH and the production of acetate and propionate linearly (P<0.001), and reduced the amount of NH₃-N in the cultures. Compared with the control, adding disodium fumarate to batch cultures reduced CH₄ production. In experiment 2, the effect of disodium fumarate on the fermentation activity of Ruminococcus flavefaciens was investigated. Disodium fumarate was added to the incubation bottles to achieve final concentrations of 0, 4, 7 and 10 mmol/L. Disodium fumarate addition increased the dry matter loss, TVFA production, and the counts of R. flavefaciens. In experiment 3, the effect of disodium fumarate on ryegrass fermentation by rumen fungi was investigated. Disodium fumarate addition (0, 4, 7, and 10 mmol/L) reduced the dry matter loss and the cumulative gas production (P<0.01). These results indicate that the effects of disodium fumarate on rumen fermentation mainly depend on the nature of the incubated substrate, with the high-forage diet showing the greatest response. Disodium fumarate addition increased the fermentation activity of the mixed rumen micro-organisms and of R. flavefaciens, but reduced the fermentation activity of the rumen fungi.