This thesis describes research to determine the effect of monensin supplementation for lactating dairy cattle on rumen methanogen diversity and quantity 'in vivo'. Fourteen cows were paired by parity and days in milk and assigned to control or monensin treatment. Rumen samples were obtained from each cow at four time points during the experimental period. Changes in methanogen diversity were analyzed using denaturing gradient gel electrophoresis; no significant affect of treatment or time was found. 'Methanobrevibacter' sp. was determined to be the predominant methanogen genus in the rumen by restriction fragment length polymorphism and sequencing. Monensin supplementation was negatively associated with methanogen per gram wet weight when quantitative real-time PCR was used but no significant treatment difference was found with fluorescent 'in situ' hybridization. This study shows that monensin does not alter the diversity of the methanogen population 'in vivo', although it may impact on the quantity of methanogens present.