We describe here a Legionella pneumophila type IV secretion system that is distinct from the previously described icm/dot system. This type IV secretion system contains 11 genes (Ivh) homologous to genes of other type IV secretion systems, arranged in a similar manner. The Ivh genes were found to be located on a DNA island with a GC content higher than the L. pneumophila chromosome. In contrast to the icm/dot system that was shown to be required for intracellular growth in HL-60-derived human macrophages and Acanthamoeba castellanii, the ivh system was found to be dispensable for Intracellular growth in these two hosts. The Ivh system was found to be partially required for RSF1010 conjugation, a process that was previously shown to be completely dependent on several icm/dot genes. However, results obtained from analysis of double mutants in the icm/dot genes and the Ivh genes revealed that Ivh genes can substitute for some components of the icm/dot system for RSF1010 conjugation, but not for intracellular growth. These results indicate that components of the icm/dot system and components of the Ivh type IV secretion system are able to interact with one another.