Legionnaires disease is an acute respiratory disease that is often fatal for immunocompromised patients. The causative agent of this disease, Legionella pneumophila, is a Gram-negative bacterium that is present in a variety of aquatic environments. L. pneumophila is a facultative intracellular parasite; it grows within human phagocytic cells and eventually causes their destruction. In contrast to many other intracellular parasites, L. pneumophila is a Gram-negative bacterium that can be grown in standard microbiological culture medium. To determine the factors that enable this organism to enter, survive, and multiply within human mononuclear phagocytes, we chose bacteriophage Mu, a powerful genetic tool that transposes within the host cell genome, to generate insertion mutations and gene fusions in the Legionella genome. Certain derivatives of Mu are able to generate fusions between target genes and the lac operon from Escherichia coli. We have determined that although Mu is unable to attach to L. pneumophila or complete its life cycle within Legionella, it does transpose within the Legionella genome. Transposition was detected with a mini-Mu phage that carries the lac operon of E. coli.