An in vitro and an in vivo study were conducted to determine the antimethanogenic activity of the macroalga Asparagopsis taxiformis (AT) in lactating dairy cows. In vitro, 9 treatments were tested in 24-h incubations with ruminal inoculum collected from 2 lactating Holstein cows. Treatments were: control (no additives), AT at 0.25, 0.5, 0.75, 1.0 and 1.5% of feed dry matter (DM), and bromoform (CHBr₃), the active compound in AT, equivalent to CHBr₃ found in AT dosed at 0.25, 1.0, and 1.5%. The in vivo study was conducted with 6 multiparous Holstein cows [68 ± 4.2 d in milk, 46 ± 2.5 kg/d milk yield (MY)]. All cows received the same AT inclusion rates of 0, [background (BG) period], 0.25, 0.5, and 0.75% of feed DM. Each phase consisted of 7-d diet adaptation and 3-d sampling, followed by a 7-d washout period. Enteric methane (CH₄) emission was measured 8 times over 3 d using the GreenFeed system. The last 4 d of each phase were used for statistical analysis of MY and DM intake (DMI). Data were analyzed using the REG (in vitro) and MIXED (in vivo; the model included treatment) procedures of SAS using contrasts. In the in vitro experiment, AT decreased CH₄ yield linearly [P < 0.001; 27.3, 35.4, 29.4, 15.0, 1.42, and 0.26 mL CH₄/g feed DM (SEM = 3.68) for control, 0.25, 0.5, 0.75, 1.0, and 1.5% AT, respectively]. CHBr₃ had no effect (P ≥ 0.59) on CH₄ production. In the in vivo experiment, DMI was decreased by the highest (0.75%) level of AT [P = 0.03; 22.7, 27.6, 25.4, and 17.7 kg/d (SEM = 1.7) for BG, 0.25, 0.5, and 0.75% AT, respectively]. MY was also decreased by 0.75% AT (P = 0.05; 46.2, 47.5, 46.0, and 39.2 kg/d respectively; SEM = 3.4). Daily CH₄ emission decreased linearly (P < 0.001) with AT inclusion rate: 392, 367, 84, and 83 g/d, (SEM = 20) respectively. CH₄ yield was decreased quadratically (P = 0.02; from 17.5 to 5.7 g/kg DMI; SEM = 1.3) and CH₄ intensity was decreased linearly (P < 0.001; from 8.4 to 2.4 g/kg milk; SEM = 3.2) by AT. In the short term in vivo experiment, inclusion of AT at 0.50% of DMI decreased CH₄ emission in lactating dairy cows by 80% and had no effect on DMI or MY.