As antibiotic-resistant bacteria spread worldwide, the need to develop novel antimicrobial agents is urgent. One rich source of potential antimicrobials is the insect immune system, as insects produce a wide range of antimicrobial peptides (AMPs) with diverse sequences and structures. Insects also encounter many bacterial pathogens, some of which are closely related to pathogens of clinical relevance. However, despite interest in AMPs as therapeutics, the relationships between the amino acid sequence, biophysical properties, antimicrobial activity, and specificity are still not generalizable. To improve our understanding of these relationships, we assessed how single amino acid changes in cecropin AMPs produced by the fruit fly,