Kassinatuerin-1 (GFMKYIGPLI(10)PHAVKAISDL(20)I.NH2) was isolated in high yield (75 nmol/g) from an extract of the skin of a Hyperoliid frog, the African running frog Kassina senegalensis and its sequence was confirmed by total synthesis. The peptide inhibited growth of the gram-negative bacterium Escherichia coli (minimum inhibitory concentration, MIC = 4 mu M), the gram-positive bacterium Staphylococcus aureus (MIC = 8 mu M), and the yeast Candida albicans (MIC = 70 mu M). A structurally related peptide, kassinatuerin-2 (FIQYLAPLI(10)PHAVKAISDL(20)I.NH2) was also isolated in high yield (96 nmol/g) from the extract but was devoid of antimicrobial activity against these microrganisms. Kassinatuerin-1 may be classified with other linear, cationic antimicrobial peptides that can potentially adopt an amphipathic alpha-helical conformation but it contains almost no amino acid sequence identity with previously characterized bioactive peptides from frog skin.