Many fiber-digesting microorganisms are strict anaerobes so fiber digestion could be decreased when ruminants swallow air during feed consumption. The objective of this study was to evaluate the effect of adding air on rumen fermentation, and to determine whether adding aerobic probiotics can ameliorate the effects of air. Twelve treatments were analyzed in a 4 × 3 factorial design with 4 levels of air treatments and 3 probiotic treatments. Air treatments included: no air added to chemically reduced medium, and 0, 25 mL or 50 mL air added to 40 mL unreduced medium with 10 mL rumen inoculum at the start of fermentation in 125-mL flasks. Probiotic treatments were: no additive (control), Dairyman’s Edge (DE), and live yeast. Timothy hay (0.5 g) and corn grain (0.5 g) were incubated at 39°C for 24 h with 5 replicates. Results were analyzed by the model: Y = μ + A + P + A×P + S + E, where Y is the response variable, and A is a fixed effect of air or reducing agent, P is the effect of yeast or probiotic additive, S is sequence, and E is error. Significant differences were accepted at P < 0.05, and tendencies at P < 0.1. Air decreased NDF digestibility from 51.8% for reduced medium to 46.1, 44.2 and 41.3% with 0, 25, and 50 mL air respectively. Air treatment tended to decrease pH from 6.41 for reduced medium to 6.25 for other treatments, but DE increased pH to 6.60 compared with 6.32 for control or yeast. There were no main effects of treatments on gas production, but there was an interaction of air treatment with additive. For yeast treatment, gas production decreased from 228 mL for reduced treatment, to 221, 170, and 108 mL for treatments with 0, 25 or 50 mL of air, while gas production for other treatments was not affected by air addition. Acetate production tended to decrease (P < 0.1) with air addition, and decreased for DE and yeast treatments. Air decreased butyrate production. Acetate to propionate ratio decreased with air or probiotic, but air did not further decrease the acetate to propionate ratio in the presence of probiotics. The addition of air to in vitro fermentation decreased NDF digestibility, and decreased expected acetate production. Use of probiotics did not ameliorate the decrease in fiber digestion due to presence of air.