Different legume sources containing varying levels of condensed tannins (CT) may alter ruminal fermentation and enteric CH₄ production. Research is needed on how increased CT levels from different legume sources may affect nutrient digestibility, volatile fatty acid (VFA) production, bacterial protein synthesis, fatty acid (FA) flow, protozoa and CH₄ production. A 4-unit continuous culture fermentor system was used to assess the effects of legume source containing varying levels of CT with orchardgrass on ruminal fermentation. Legumes were: alfalfa (ALF; control); birdsfoot trefoil (BFT; low CT); crown vetch (CV; intermediate CT); and sericea lespedeza (SL; high CT). Treatments consisted of orchardgrass:legume (50:50). The experimental design was a 4 × 4 Latin square. Forage, effluent and microbial samples were analyzed for dry matter (DM), organic matter (OM), crude protein (CP), acid detergent fiber (ADF) and neutral detergent fiber (NDF) to determine nutrient digestibilities. Effluent samples were taken for VFA concentrations, bacterial protein synthesis, FA flow and protozoa count. Gas samples for CH₄ analysis were recorded by a photoacoustic gas analyzer. Data were analyzed by PROC-GLIMMIX of SAS. Apparent and true DM and OM digestibilities were lower (P < 0.01) in SL than ALF and BFT, with CV in an intermediate level. Apparent NDF and ADF digestibilities were lowest (P < 0.01) in SL. Total VFA concentrations were least (P < 0.001) in SL and greatest in ALF and BFT. Greater dietary N (P < 0.05) and lower NH₃-N (P < 0.001) were found in SL and BFT than in ALF and CV. Bacterial N efficiencies per kg of truly digested DM and OM were least (P < 0.05) in SL and greatest in BFT. Changes in FA biohydrogenation process were found with SL due to decreases in conjugated linoleic acid flow and in the ratio between stearic and vaccenic acids. The SL had the greatest numbers of protozoa (P < 0.05), the least total CH₄ production (P < 0.001) and the least CH₄ production per unit of OM truly digested (P < 0.001) which offset increased protozoal number. In summary, introducing SL into an orchardgrass diet reduced CH₄ production but exerted adverse effects on ruminal feed digestion and fermentation.