Our objective was to evaluate the effect of a multispecies fungal extract (MFE) on growth performance, apparent total tract digestibility (ATTD), fermentation characteristics, and rumen microbiome composition of beef cattle fed forage-based diets. For experiment 1, ruminally cannulated Angus × SimAngus cows (n = 4; body weight [BW] = 569 ± 21 kg) were used in a randomized crossover design with two 21-d study periods and a 23-d washout period to evaluate the effect of dietary inclusion of an MFE on in situ digestion, ruminal fermentation, and the composition of the rumen microbiome. Treatments consisted of a forage-based diet with or without the inclusion of a MFE. Rumen samples were collected on days 5, 10, and 20. Experiment 2 evaluated different inclusion rates of the MFE in a randomized complete block design using Angus × SimAngus-crossbred steers (n = 80; BW = 370 ± 44 kg). Steers were blocked by BW and randomly assigned to one of four treatments (2 pens/treatment): diet with no MFE, 0.02%, 0.04%, and 0.08% of the MFE (dry matter [DM] basis). Steers were fed a forage-based diet for 122 d. Subsets of 10 steers/treatment were randomly selected for the determination of ATTD on d 20, 40, and 60. All data were analyzed using the MIXED procedure of SAS. In exp 1, adding the MFE to the diet tended to increase the ruminal disappearance rate of the DM on day 10 (P = 0.06). No interactions or treatment effects were observed for the short-chain fatty acid profile of the rumen fluid (P ≥ 0.13). Metagenomic analysis of the rumen microbiome showed an MFE × d interaction for the Fibrobacter genus (P = 0.01), which on day 20 was less abundant in the rumen of cows fed the MFE. In exp 2, steers supplemented with 0.04% of MFE had a lower average daily gain and were lighter at the end of the experiment (cubic, P ≤ 0.04) compared to steers supplemented with 0.02% MFE. Steers fed the diet with 0.02% of MFE had the greatest gain-to-feed ratio among the MFE-supplemented groups (cubic, P < 0.01). Dietary inclusion of the MFE increased neutral detergent fiber digestibility (linear, P = 0.05). Steers supplemented with 0.04% of MFE had the greatest acid detergent fiber digestibility among treatments (quadratic, P = 0.03). Collectively, results showed that ruminal disappearance rate and digestibility of forage-based diets increased due to MFE supplementation, but did not translate into growth performance improvements or beneficially alter rumen fermentation.