The objective of this study was to measure feedlot performance and carcass characteristics of heifers fed a newly developed direct-fed microbial (DFM) or not within 2 different basal diets. Three hundred thirty-six heifers (348 ± 33 kg) were used in a randomized block designed study with 24 pens (6 replications/treatment). The treatment design was a 2 × 2 factorial arrangement consisting of 2 basal diets based on corn (CON) or 40% modified distillers grains plus solubles replacing corn (MDGS) and presence (1 × 10⁹ cfu/mL of each bacteria species/animalper day) or absence of DFM. The DFM was isolated from prior experiments, grown in the lab, and consisted of members of Bacteroides and Lachnospiraceae that were top-dressed on feed immediately after feeding to ensure accurate supply daily to pens. One block (4 pens) of heifers was harvested after 133 d on feed and 3 blocks (20 pens) were harvested after 147 d on feed. Hot carcass weight was obtained the day of harvest, and LM area and fat thickness collected after a 48-h chill. Carcass adjusted final BW was calculated using HCW divided by a common dressing percentage (63%), and used for ADG and G:F. All data were analyzed using the MIXED procedure of SAS with an α = 0.05 and pen as the experimental unit. There were no interactions (P > 0.25) between diet and DFM supplementation for DMI, ADG, G:F, and HCW; thus, main effects of basal diet and DFM are presented. There were no differences in DMI, ADG, and carcass characteristics (P > 0.23) due to supplementation of DFM. Heifers fed DFM tended (P = 0.10) to be less efficient. Feeding MDGS increased (P < 0.01) DMI, ADG, and HCW of heifers. Feed efficiency was improved (P < 0.01) by 7% for heifers fed MDGS diets compared with CON. Heifers fed MDGS were fatter (P < 0.01) compared with CON likely due to greater ADG. The DFM developed for this study did not enhance performance, while feeding modified distillers grains compared with corn improved performance.