The final step of the intracellular life cycle of Legionella pneumophila and other intracellular pathogens is their egress from the host cell after termination of intracellular replication. We have previously isolated five spontaneous mutants of L. pneumophila that replicate intracellularly similar to the wild-type strain but are defective in pore formation-mediated cytolysis and egress from mammalian and protozoan cells, and the mutants have been designated rib (release of intracellular bacteria). Here, we show that the rib mutants are not defective in the activity of enzymes secreted through the type II secretion system, including phospholipase A, lysophospholipase A, and monoacylglycerol lipase, although they are potential candidates for factors that lyse host cell membranes. In addition, the pilD and lspG mutants, which are defective in the type II secretion system, are not defective in the pore-forming toxin. We show that all five rib mutants have an identical point mutation (deletion) following a stretch of poly(T) in the icmT gene. Spontaneous revertants of the rib mutants, due to an insertion of a nucleotide following the poly(T) stretch in icmT, have been isolated and shown to have regained the wild-type phenotype. We constructed an icmT insertion mutant (AA100kmT) in the chromosome of the wild-type strain by allelic exchange. The AA100kmT mutant was as defective as the rib mutant in pore formation-mediated cytolysis and egress from mammalian and protozoan cells. Both the rib mutant and the AA100kmT mutant were complemented by the icmT gene for their phenotypic defect. rtxA, a gene that is thought to have a minor role in pore formation, was not involved in pore formation-mediated cytolysis and egress from mammalian and protozoan cells. We conclude that the icmT gene is essential for pore formation-mediated lysis of mammalian and protozoan cells and the subsequent bacterial egress.