The objective of this study was to investigate the effect of active-dry yeast (ADY; Vistacell, Marlborough, UK) and a combination of ADY and a calcified seaweed product (4% Vistacell AB, AB Vista) on rumen fermentation characteristics. Three ruminally-fistulated dairy cows (49 ­ ± 3 DIM) were used in a 3 × 3 Latin square arrangement. The treatments were: T1 (100 g/d limestone), T2 (4 g/d of ADY + 96 g/d of limestone, and T3 (4 g/d of ADY + 88 g/d of AB + 8 g/d of limestone). The cows were fed ad libitum 2×/d a 65:35 (F:C; DM basis). Each period lasted 3 wks, a 2-wk adaptation phase followed by 1 wk of measurements. Throughout the experiment, milk yield and DMI were measured daily. Ruminal pH was recorded every minute from d 15 through d 21 using an indwelling electrode system. Apparent total-tract digestibility was measured during d19 to d21. Rumen samples were collected during d19 to d21 at 0, 2, 4, 6 and 8 h after AM feeding and analyzed for VFA and ammonia content. Milk components were analyzed during d19 to d21 for protein, fat content, and somatic cell count. Statistical analysis was conducted using ANOVA. Results showed that T2 and T3 increased (P < 0.05) 4% FPCM yield compared with T1 (37.1 kg/d) by 1.5 and 1.8kg/ day respectively. DMI was increased by T2 by 0.8 kg/day (P < 0.05) compared with T1 and T3 (23.4 vs 23.6 kg/d, respectively). There was no significant effect on feed efficiency between the 3 groups (1.59 vs 1.60 vs 1.65; T1, T2, T3, respectively). Mean daily ruminal pH was numerically higher (pH 6.28 ­ ± 0.5) for both T2 and T3 than T1 (pH 6.22 ­ ± 0.7). Time spent below pH 5.8 was 159, 87, and 50 min/d for T1, T2, and T3, respectively. Different fermentation profiles were observed, indicating a shift in acetate:propionate:butyrate (mol/100 mmol) with T3 (63.1: 20.2: 13.6), T1(61.9: 22.2: 13.0) and T2 61.4:22.8: 12.8) with T3 showing an increase in acetate and butyrate (P < 0.05) and reduction in propionate (P < 0.05). Propionate was increased in T2 (P < 0.05). No significant effects were observed on digestibility. The results showed that ADY can positively affect rumen fermentation and animal performance. Different effects are observed when in combination with calcified seaweed