The potential of essential oils (EO) from leaves of goat weed (EOG) and African basil (EOB) to reduce in vitro enteric methane production and improve rumen fermentation was examined. A corn silage-based TMR (0.5 g; CP 16.6%; NDF 35.9%) was treated with EOG or EOB at rates of 0 (Control), 10 (Low), 20 (Med) and 30 (High) μL/50 mL of rumen fluid - buffer inoculum (ratio 1:2) and with monensin (0.6 mg/50 mL). Each suspension was incubated in a 120-mL gas-tight culture bottle in triplicate at 39°C for 24 h in each of 2 runs. Fermentation parameters, gas and methane production, in vitro DM digestibility (DMD), and fermentation efficiency (DMD g kg⁻¹/gas volume) were measured. Data for each EO were separately analyzed with the Glimmix procedure of SAS. Compared with respective Controls, monensin and High EOG or EOB reduced (P < 0.05) gas volume (66.8, 71.7 and 49.3 vs. 85.0), DMD (526, 520, and 514 vs. 555 g/kg) and methane percentage in total gas (7.95, 9.02 and 7.19 vs. 10.4%) and increased (P < 0.05) fermentation efficiency (7.92, 7.39, and 11.58 vs. 6.58). The respective reductions in methane production (mg/g DM digested) were 36.9, 24.1, 57.2%. Ammonia nitrogen concentration, pH of EOB and VFA concentrations of EOG were unaffected (P > 0.05) by treatment. However, monensin and High EOG increased (P < 0.05) the pH (5.75, 5.75 vs. 5.66, respectively) and monensin and High EOB increased (P < 0.05) molar proportions of butyrate and High EOB decreased (P < 0.05) that of propionate. Low rates of EOB and EOG increased (P < 0.05) CH4 production (mg/g DM digested) and Med rates decreased (P < 0.05) DMD. Low EOG increased (P < 0.05) gas volume and methane production (mg/g DM digested), and decreased (P < 0.05) fermentation efficiency. A high dose of essential oils from goat weed and African basil leaves improved fermentation efficiency and reduced DMD and methane production in a manner that is comparable with monensin.