Increasing concern over the role of greenhouse gases in global warming has lead to a renewed interest in the production of methane by ruminants. Sheep are routinely used to study digestibility, however their use as a model to study methane production by cattle has received only limited attention (Blaxter and Wainman, 1964). The objective of the current study was to establish whether differences in methane production exist between sheep and cattle and to measure the magnitude of these differences under various dietary situations.