ABSTRACT is the predominant cause of Legionnaires' disease in the United States and Europe, while is the common cause of the disease in Western Australia. Although clinical manifestations by both intracellular pathogens are very similar, recent studies have shown that phagosome biogeneses of both species within human macrophages are distinct (R. Asare and Y. Abu Kwaik, Cell. Microbiol., in press). Most inbred mouse strains are resistant to infection by , with the exception of the A/J mouse strain, and this genetic susceptibility is associated with polymorphism in the allele and flagellin-mediated early activation of caspase 1 and pyropoptosis in nonpermissive mouse macrophages. Here, we show that genetic susceptibility of mice to infection by is independent of allelic polymorphism of replicates within bone marrow-derived macrophages and in the lungs of A/J, C57BL/6, and BALB/c mice, while replicates in macrophages in vitro and in the lungs of the A/J mouse strain only. Quantitative real-time PCR studies on infected A/J and C57BL/6 mouse bone marrow-derived macrophages show that both and trigger similar levels of expression, but the levels are higher in infected C57BL/6 mouse macrophages. In contrast to , has no detectable pore-forming activity and does not activate caspase 1 in A/J and C57BL/6 mouse or human macrophages, despite flagellation. Unlike , triggers only a modest activation of caspase 3 and low levels of apoptosis in human and murine macrophages in vitro and in the lungs of infected mice at late stages of infection. We conclude that despite flagellation, infection by is independent of polymorphism in the allele and does not trigger the activation of caspase 1, caspase 3, or late-stage apoptosis in mouse and human macrophages. Neither species triggers caspase 1 activation in human macrophages.