One of the characteristics of the host defense of insects is the rapid synthesis of a variety of potent antibacterial and antifungal peptides. To date, seven types of inducible antimicrobial peptides (AMPs) have been characterized in Drosophila. The importance of these peptides in host defense is supported by the observation that flies deficient for the Toll or Immune deficiency (Imd) pathway, which affects AMP gene expression, are extremely susceptible to microbial infection. Here we have developed a genetic approach to address the functional relevance of a defined antifungal or antibacterial peptide in the host defense of Drosophila adults. We have expressed AMP genes via the control of the UAS/GAL4 system in imd; spätzle double mutants that do not express any known endogenous AMP gene. Our results clearly show that constitutive expression of a single peptide in some cases is sufficient to rescue imd; spätzle susceptibility to microbial infection, highlighting the important role of AMPs in Drosophila adult host defense.