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Background:This study investigated changes in rumen protozoal and methanogenic communities, along with the correlations among microbial taxa and methane (CH4) production of six Belmont Red Composite beef steers fed tea seed saponins (TSS). Animals were fed in three consecutive feeding periods, a high-grain basal diet for 14 d (BD period) then a period of progressive addition of TSS to the basal diet up to 30 g/d for 20 d (TSS period), followed by the basal diet for 13 d without TSS (BDP post-control period). Results:The study found that TSS supplementation decreased the amount of the protozoal genus Entodinium and increased Polyplastron and Eudiplodinium genera. During BDP period, the protozoa community of steers did not return to the protozoal profiles observed in BD period, with higher proportions of Metadinium and Eudiplodinium and lower Isotricha. The addition of TSS was found to change the structure of methanogen community at the sub-genus level by decreasing the abundance of methanogens in the SGMT clade and increasing the abundance of methanogens in the RO clade. The correlation analysis indicated that the abundance of SGMT clade methanogens were positively correlated with Isotricha, and Isotricha genus and SGMT clade methanogens were positively correlated with CH4 production. While RO clade were positively correlated with the proportion of Metadinium genus, which was negatively correlated with CH4 emission. Conclusions:These results suggest that different genera of rumen protozoa ciliates appear to be selectively inhibited by TSS, and the change in methanogen community at the subgenus level may be due to the mutualistic relationships between methanogens and rumen ciliates.