Reported here is the isolation and characterization of two antibacterial peptides synthesized in an ant Myrmecia gulosa in response to bacterial challenge. The peptides were purified by reversed-phase high performance liquid chromatography and characterized by peptide sequencing and mass spectrometry. Both peptides were formed from 16 amino acids, were rich in proline (~30%), and had N-acetylgalactosamine O-linked to a conserved threonine. The activity of a synthetic non-glycosylated isoform was markedly reduced demonstrating that glycosylation was necessary for maximum activity. The peptides were active only against growing Escherichia coli. They were inactive against stationary cells, Gram-positive bacteria, the yeast Candida albicans, two species of mammalian cells, and bovine pestivirus.