It is unknown how Legionella pneumophila cells escape the degradative lysosomal pathway after phagocytosis by macrophages and replicate in an organelle derived from the endoplasmic reticulum. Here we show that, after internalization, L. pneumophila-containing phagosomes recruit early secretory vesicles. Once L. pneumophila phagosomes have intercepted early secretory vesicles they begin to acquire proteins residing in transitional and rough endoplasmic reticulum. The functions of Sar1 and ADP-ribosylation factor-1 are important for biogenesis of the L. pneumophila replicative organelle. These data indicate that L. pneumophila intercepts vesicular traffic from endoplasmic-reticulum exit sites to create an organelle that permits intracellular replication and prevents destruction by the host cell.