Peptidomic analysis of norepinephrine-stimulated skin secretions from Hose's rock frog Odorrana hosii (Boulenger, 1891) led to the isolation of 8 peptides with differential antibacterial activities. Structural characterization demonstrated that the peptides belonged to the esculentin-1 (1 peptide), esculentin-2 (1 peptide), brevinin-1 (2 peptides), brevinin-2 (2 peptides), and nigrocin-2 (2 peptides) families of antimicrobial peptides. Similar analysis of skin secretions from the Malaysian fire frog Hylarana picturata (Boulenger, 1920) led to the isolation and characterization of peptides belonging to the brevinin-1 (2 peptides), brevinin-2 (5 peptides), and temporin (1 peptide) families. The differences in antimicrobial activities of paralogous peptides provide insight into structure-activity relationships, emphasizing the importance of cationicity in determining potency. The substitution Lys11-->Gln in brevinin-1HSa (FLPAVLRVAAKIVPTVFCAISKKC) from O. hosii abolishes growth inhibitory activity against Escherichia coli but has no effect on the high potency (MIC = 8 microg/ml) against Staphylococcus aureus. In contrast, the substitution (Gly4-->Asp) in brevinin-2PTb (GFKGAFKNVMFGIAKSAGKSALNALACKIDKSC) from H. picturata reduces activity against both E. coli and S. aureus. Cladistic analysis based upon the amino acid sequences of the brevinin-2 peptides from Asian frogs provides evidence for sister taxon relationships between O. hosii and O. livida and between H. picturata and H. güntheri.