This study evaluated the effects of supplementing yeast, probiotic bacteria, or a combination on rumen fermentation and nutrient digestion in continuous rumen fermentors. Rumen fluid (10 L) from a mid-lactation fistulated Holstein cow was collected and used to inoculate eight 1-L glass continuous culture fermenters. Fermentors were randomly assigned to 1 of 4 dietary treatments in a 2 × 2 factorial arrangement, including: Control, no supplement; Yeast, supplemented with 3.6 × 10⁷ cfu/d Saccharomyces cerevesiae; Probiotic, supplemented 5.4 × 10⁶ cfu/d of a blend of Enterococcus faecium and Lactobacillus casei; or Combination, supplemented with both the Yeast and Probiotic treatments. The study was conducted in 2 blocks with 8 fermentors / block (n = 4). Fermentors were fed 20 g twice daily. Artificial saliva was added for a 6% dilution/h. Samples were collected from each fermentor on d 5 and 7 at 0, 2, 4, 6, 8, and 12 h relative to the AM feeding and pH recorded and samples collected for volatile fatty acids. On d 5, 6, and 7 total collection of the 24-h effluent was used to determine digestibility of DM and NDF. Data were analyzed as a repeated measures for a 2 × 2 factorial design using the Mixed procedure of SAS with period included as a random effect. Data are reported as Control, Yeast, Probiotic, and Combination throughout. There were no treatment × time or treatment effects (P ≥ 0.582) on pH (6.24, 6.23, 6.19, and 6.21 ± 0.081). Additionally, there were no treatment × time or treatment effects on concentrations of acetate (P ≥ 0.237; 62.2, 63.7, 64.6, and 63.8 ± 0.99 mM), propionate (P ≥ 0.263; 24.6, 25.8, 24.9, and 26.0 ± 1.48 mM), or total volatile fatty acids (P ≥ 0.450; 109.7, 113.4, 113.1, and 114.6 ± 4.78 mM). There was a tendency for yeast supplementation (P = 0.067; 51.5, 56.8, 53.1, and 54.1 ± 1.54%) to increase DM digestion. There was no treatment effects on digestion of NDF (P ≥ 0.715; 56.9, 59.6, 59.0, and 59.0 ± 4.07%). These data indicate that supplementing active dry yeast may improve DM digestion.