Livestock are one of the largest single sources of methane emission, equivalent to 15-20 % of total anthropogenic methane. Selective suppression of the rumen protozoa has been suggested to be promising approach to reduce methane release (Moss et al., 2000) as up to 25 % of the methanogens in the rumen are associated with protozoa (Newbold et al., 1995). However, contradictory results have been reported between in vitro and in vivo data and short and long term defaunation experiments (Ranilla et al., 2003). This study was carried out to investigate the effect of the absence of protozoa in the rumen on enteric methane production by lambs.