Insect defensin A is an inducible antibacterial peptide of the fleshflyPhormia terra-novaeand has been recently characterized as a 40 residue basic peptide with six cysteines engaged in three intramolecular disulfide bridges. We report the expression of this peptide inSaccharomyces cerevisiaeas a fusion protein carrying at its N-terminus leader sequences which are derived from the precursor of the yeast pheromone mating factor α (MFαl). These sequences allow the biologically active peptide to be secreted at high levels in a correctly processed form. Thus heterologous production of defensin A circumvents laborious purification of minute amounts of the peptide from its natural source. The insect defensin pro peptide can substitute for theMFαlpro peptide, indicating that yeast can process an insect pro sequence.