Decreasing methanogenesis without affecting fermentation and digestion of feeds in the rumen can reduce the environmental impact of ruminant production and have a beneficial effect on feed conversion efficiency. In this work, metabolites produced by Monascus spp. molds were assayed for their antimethanogenic activity in vitro and in vivo. The capacity of 7 strains of Monascus to produce secondary metabolites was assessed in solid media. Monitored metabolites included the statins monacolin K, pravastatin, and mevastatin, and the mycotoxin citrinin. Ethanolic extracts from 5 different solid media from 2 selected strains were tested in vitro. Fermentation was not negatively affected by any treatment, but one extract decreased methane production (P < 0.05). This extract was further assayed in 3 consecutive batch incubations where a marked decrease in methane was observed in the third batch (P < 0.05). In contrast, methane produced in flasks with pure monacolin K was not different from controls (P > 0.05). Rice on which the selected Monascus sp. was grown also decreased methane production when used as substrate for in vitro incubations (P < 0.05). The effect of Monascus-fermented rice on methane production was then assayed in vivo. Six wethers were adapted to a diet containing rice grain and hay (1:1 ratio). Rice was then replaced by fermented rice and given to animals for nearly 2 wk. Animals were monitored for a further 2 wk after the treatment. Daily methane emissions decreased (P < 0.05) by 30% after 2 to 3 d into the treatment and remained low throughout the administration period. This change was associated with reduced ruminal acetate to propionate ratio and decreased numbers of methanogens as detected by quantitative PCR (P < 0.05). In contrast, no changes in the methanogenic community were observed by denaturing gradient gel electrophoresis (DGGE). Total bacteria numbers increased (P < 0.05) with changes in the DGGE profile community, whereas protozoa were not affected by the treatment. Methane emissions and the acetate to propionate ratio remained numerically less in the 2 wk posttreatment as compared with measures before treatment. Metabolites produced by Monascus appear to have an inhibitory effect on methanogens and decreased methanogenesis in vitro and in short-term in vivo without any apparent negative effect on rumen fermentation. This strategy deserves to be further explored and could be an abatement option under certain feeding situations.