This study evaluated Saccharomyces cerevisiae fermentation products (Diamond V original XPC and 2 prototypes) on lactational performance and ruminal fermentation. Eight ruminally cannulated (132 DIM and 34.4 kg milk) Holstein dairy cows (2 primiparous and 6 multiparous) were blocked by milk yield, DIM and parity and randomly assigned in a replicated 4 × 4 Latin square design. Treatments were (1) Control (C): corn silage and haylage based ration; (2) XPC: C ration with 14 g/hd/d Original XPC; (3) Prototype 1 (P1): C ration with 5 g/hd/d P1; and (4) Prototype 2 (P2): C ration with 19 g/hd/d P2. Treatments were mixed with dried distillers grains and then mixed in the TMR at 454 g/hd/d. Periods were 28 d with the first 21 d for adjustment followed by 7 d of data collection. Milk yield (3×/d) was recorded d and milk samples (2 d) during wk 4. On d 25 or 27, rumens were evacuated, weighed, markers added (Co and valeric acid), mixed, block the rumen-omasal orifice using a sponge, and rumen contents returned to the rumen. Ruminal samples were collected for 4 h at 20 min intervals to determine ruminal pH, ammonia, and volatile fatty acid concentrations. After 4 h of sample collection, rumen contents were re-evacuated, re-weighed, rumen-omasal sponge removed, and rumen contents returned. One cow died unrelated to study objectives and data were removed. Milk yield (30.7, 32.3, 32.0, 31.3 kg/d for C, XPC, P1, and P2, respectively) and intake of DM [(DMI); 24.5, 23.6, 23.6, and 25.3 kg/d] were similar (P > 0.10) between all cows, but feed efficiency (1.26, 1.36, 1.36, and 1.24 kg/kg milk/DMI) and energy-corrected milk/DMI (1.42, 1.54, 1.52, and 1.38 kg/kg was greater (P < 0.01) for cows fed XPC and P1 compared with cows fed C and P2. Milk composition was similar (P > 0.10) between cows fed all rations. Ruminal pH (6.06, 6.07, 6.02 and 6.13) was greater (P < 0.05) for cows fed P2 compared with cows fed other rations. The feeding of a dairy ration with a Saccharomyces cerevisiae fermentation product can improve ruminal pH and feed efficiency of mid-lactation cows.