The effects of Saccharomyces cerevisiae fermentation products (SCFP) on the apparent digestibilities of dry matter (DM), neutral detergent fiber (NDF) and phosphorus (P) in lactating cows during control feeding and during grain-based subacute ruminal acidosis (SARA) challenges were investigated. Thirty-two Holstein lactating dairy cows were assigned to 4 treatments, i.e., control, and 3 different SCFP supplementations. Cows in the 3 SCFP treatment groups received 14 g/d Diamond V Original XPC (XPC), 19 g/d NutriTek (NTL), or 38 g/d NutriTek (NTH) mixed with 126, 121, and 102 g/d ground corn, respectively, while the cows in the Control group received 140 g/d ground corn only. Supplements were top dressed once daily immediately after feed delivery from 4 wk pre-calving to 11 wk post-calving. At wk 5 and 8 after calving one-week grain-based SARA challenges were conducted by switching from a lower to a higher concentrate diet (50% to 70% concentrate, DM basis). Diet samples were collected weekly and fecal samples of individual cows were collected twice weekly. Samples were pooled for wk 1–4 after calving (preSARA), wk 5 after calving (first SARA challenge), and wk 8 after calving (second SARA challenge). Samples were analyzed for DM, acid insoluble ash (AIA), NDF and P (% DM basis). Apparent total-tract digestibilities for DM, NDF and P were calculated using AIA as an internal marker. The apparent total-tract digestibility of DM and P were not affected by the SARA challenges and SCFP, and averaged 68.9 and 52.6%, respectively, across treatments and weeks. The SARA challenges reduced the apparent total-tract digestibility of NDF from 61.1 to 49.0% (P < 0.01), but the NTH supplementation increased NDF digestibility from 52.7 to 61.8% (P < 0.02). Our results show that SCFP can increase fiber digestion, which is particularly important during high grain feeding.