<jats:p>Hemocytes from the invertebrate <jats:italic>Styela clava</jats:italic>, a solitary tunicate, contained a family of four α‐helical antimicrobial peptides that were purified, sequenced and named clavanins A, B, C and D. Each clavanin contained 23 amino acid residues and was C‐terminally amidated. The tunicate peptides resembled magainins in size, primary sequence and antibacterial activity. Synthetic clavanin A was prepared and displayed comparable antimicrobial activity to magainins and cecropins. The presence of α‐helical antimicrobial peptides in the hemocytes of a urochordate suggests that such peptides are primeval effectors of innate immunity in the vertebrate lineage.