Objectives To identify and characterize a novel antimicrobial peptide, catesbeianin-1. Results Catesbeianin-1 is 25 amino acids long and is alpha-helical, cationic and amphipathic. It had antimicrobial activity against Gram-positive and Gram-negative bacteria. It was resistant against trypsin and pepsin. Catesbeianin-1 exhibited moderate hemolytic activity (approx 8%) at 100 mu g/ml, and its HC50 ( 50% hemolytic concentration) was 300 mu g/ml. Its cytotoxicity was approx 10-20% at 100 mu g/ml, and its CC50 (50% cytotoxic concentration) was > 100 mu g/ml. The LD50 of catesbeianin-1 in mice was 80 mg/kg. At 3.1 mu g/ml, catesbeianin-1 significantly inhibited the growth of methicillin-resistant Staphylococcus aureus. Conclusions A new antimicrobial peptide from the skin of Lithobates catesbeianus (American bullfrog) may represent a template for the development of novel antimicrobial agents.