In holometabolous insects including Drosophila melanogaster a wave of autophagy triggered by 20-hydroxyecdysone is observed in the larval tissues during the third larval stage of metamorphosis. We used this model system to study the genetic regulation of autophagy. We performed a genetic screen to select P-element insertions that affect autophagy in the larval fat body. Light and electron microscopy of one of the isolated mutants (l(3)S005042) revealed the absence of autophagic vesicles in their fat body cells during the third larval stage. We show that formation of autophagic vesicles cannot be induced by 20-hydroxyecdysone in the tissues of mutant flies and represent evidence demonstrating that the failure to form autophagic vesicles is due to the insertion of a P-element into the gene coding SNF4Agamma, the Drosophila homologue of the AMPK (AMP-activated protein kinase) gamma subunit. The ability to form autophagic vesicles (wild-type phenotype) can be restored by remobilization of the P-element in the mutant. Silencing of SNF4Agamma by RNAi suppresses autophagic vesicle formation in wild-type flies. We raised an antibody against SNF4Agamma and showed that this gene product is constitutively present in the wild-type larval tissues during postembryonal development. SNF4Agamma is nearly absent from the cells of homozygous mutants. SNF4Agamma translocates into the nuclei of fat body cells at the onset of the wandering stage concurrently with the beginning of the autophagic process. Our results demonstrate that SNF4Agamma has an essential role in the regulation of autophagy in Drosophila larval fat body cells.