The function of phosphatidylcholine (PC) in the bacterial cell envelope remains cryptic. We show here that productive interaction of the respiratory pathogen Legionella pneumophila with host cells requires bacterial PC. Synthesis of the lipid in L. pneumophila was shown to occur via either phospholipid N-methyltransferase (PmtA) or phosphatidylcholine synthase (PcsA), but the latter pathway was demonstrated to be of predominant importance. Loss of PC from the cell envelope caused lowered yields of L. pneumophila within macrophages as well as loss of high multiplicity cytotoxicity, while mutants defective in PC synthesis could be complemented either by reintroduction of PcsA or by overproduction of PmtA. The lowered yields and reduced cytotoxicity in mutants with defective PC biosynthesis were due to three related defects. First, there was a poorly functioning Dot/Icm apparatus, which delivers substrates required for intracellular growth into the cytosol of infected cells. Second, there was reduced bacterial binding to macrophages, possibly due to loss of PC or a PC derivative on the bacterium that is recognized by the host cell. Finally, strains lacking PC had low steady-state levels of flagellin protein, a deficit that had been previously associated with the phenotypes of lowered cytotoxicity and poor cellular adhesion.