Dermaseptin is a water-soluble, thermostable, and nonhemolytic peptide endowed with highly potent antimicrobial activity against pathogenic fungi at micromolar concentration. Circular dichroism spectra of dermaseptin in hydrophobic media indicated 80% alpha-helical conformation, and predictions of secondary structure suggested that dermaseptin can be configured as an amphiphatic alpha-helix spanning over residues 1-27, a structure that perturbs membrane functions regulating water flux. Dermaseptin exerts a lytic action upon bacteria, protozoa, yeasts, and filamentous fungi at micromolar concentrations. Molecular elements responsible for the exceptional antimicrobial potency of dermaseptin are to be traced to the NH2-terminal alpha-helical amphipathic segment spanning residues 1-18 of the molecule. Dermaseptin (5-100 μg/ml; 48 hours) inhibits by 100% the proliferation of most microorganisms tested, including Gram-positive or Gram-negative bacteria, parasites, yeasts, and filamentous fungi, at micromolar concentrations. Dermaseptin (5-100 μg/ml; 48 hours) does not inhibit the proliferation of human KJ3 cells after a 48 h incubation, and dermaseptin treatment for 1 h does not permeate guinea pig lymphocytes up to the highest concentration assayed (200 μg/ml). Hemolysis of rabbit erythrocytes occurrs after 1 h of treatment at doses above 200 μg/ml, with 50% hemolysis at 350 μg/ml. Dermaseptin has antimicrobial activities and is against Aeromonas cauiae , Pseudomonas aeroginusa , Escherichia coli , Enterococcus faecalis , L. mezicana (NF ^α strain) and Microsporum canis (IP1194) with MIC values of 50 μg/ml; 100 μg/ml; 25 μg/ml; 15 μg/ml; and 50 μg/ml, respectively.