A trial was conducted in a commercial feedlot in northeastern Mexico to determine the effects of supplemental virginiamycin (Vm) in combination with monensin (Mn) on finishing cattle performance. Upon arrival, 2,256 crossbred heifers (BW = 276.0 ± 3.33 kg) were dewormed, vaccinated against respiratory and clostridial pathogens and implanted with a trenbolone acetate (TBA) implant (200 mg TBA + 20 mg estradiol benzoate). At processing, animals were allotted to 22 pens (approx. 100/pen) using a randomized complete block design. Two treatment groups were evaluated, both receiving a high-concentrate (+90%) diet. One treatment group (MN) had the basal diet supplemented with 400 mg/hd/d of Mn; the other treatment group (VM) diet was supplemented with 250 mg/hd/d of Vm and 300 mg/hd/d of Mn. Cattle were gradually adapted to their final diet (15.5% protein, 1.51 Mcal NEg/kg) over a 21–28 d period. When cattle had 44 d on feed (DOF), they were reimplanted with the same TBA implant. Zilpaterol chlorhydrate was provided to all cattle at 0.15 mg/kg BW for 30 d and was withdrawn 4 d before harvest. Cattle were harvested after 100 DOF. At harvest, HCW was recorded. Data were analyzed using the PROC MIXED analysis of SAS for a randomized complete block design. Pen was used as the experimental unit and block was included in the model as a random factor. After 44 DOF when cattle were reimplanted, the VM group had greater ADG (1.51 vs. 1.38; P < 0.05). At harvest, total ADG was 6% greater in the VM group (1.40 vs. 1.32; P < 0.05), and G:F was improved 7% in the VM group (0.179 vs. 0.167; P < 0.05). Hot carcass weight was 4.9 kg greater (P < 0.01) in the VM group than in the MN group (251.7 kg vs. 246.8 kg). These results agree with previous research in commercial feedlots in Mexico and Brazil. Under the parameters of this study, virginiamycin inclusion in a feedlot diet containing monensin improved feedlot performance and carcass weight.