SUMMARY. The individual and combined effects of feeding fumonisin B₁ (FB₁; 0, 100, 200 mg FB₁/kg) and moniliformin (M; 0, 100, 200 mg M/kg) were evaluated using a 3 × 3 factorial arrangement of treatments. Significant mortality (P < 0.05) occurred in chicks fed all diets containing 200 mg M/kg (50%–65%). Compared with controls and chicks fed FB₁, both feed intake and body weight gain were decreased (P < 0.05) in chicks fed diets containing 100 mg M/kg. Chicks fed M had heavier heart weights (P < 0.05) than control chicks or chicks fed FB₁. Compared with controls, chicks fed diets containing 200 mg M/kg or a combination of 200 mg FB₁/kg and 100 mg M/kg had increased kidney and liver weights (P < 0.05). Significant FB₁ by M interactions (P < 0.05) were observed for serum total protein and aspartate aminotransferase. Mild to moderate periportal extramedullary hematopoiesis and mild focal hepatic necrosis were observed in chicks fed FB₁ alone. An increased incidence of large pleomorphic cardiomyocyte nuclei, loss of cardiomyocytes, and mild focal renal tubular mineralization were observed in chicks fed M alone. Both cardiac and renal lesions were observed in chicks fed combinations of FB₁ and M. Data indicate FB₁ and M, alone or in combination, can adversely affect chick performance and health at these dietary concentrations. The interactive effects of FB₁ and M were not synergistic and were less than additive in nature. At the dietary concentrations studied, M is much more toxic to broilers than FB₁.