Organic acids, including malic and fumaric acids, have been shown to decrease methane formation from ruminal fermentation. Although they have been found to be effective hydrogen sinks, whereby H2 is diverted away from methane towards propionate formation, organic acids are problematic in practice because their acidic properties restrict the quantity which can be fed. The present study was undertaken to determine the influence of encapsulating fumaric acid on its ability to decrease methane formation and to assess the usefulness of encapsulated fumaric acid in a lamb feeding trial. Fumaric acid encapsulated in a shell of hydrogenated vegetable oil caused no drop in pH when added to ruminal fluid in vitro, but it retained its suppression of methane formation. Growing lambs on a control concentrate diet with ad libitum straw produced 23.9 L/day of methane. Dietary fumaric acid and encapsulated fumaric acid (10% of diet) decreased methane formation to 12.2 and 6.0 L/day respectively. Live weight gain over 43 days was 182, 168 and 202 g/day, while feed conversion ratios were 108, 119 and 132 g gain/kg feed intake, respectively.