Intracellular parasites of the genus Leishmania generate severe diseases in humans, which are associated with a failure of the infected host to induce a protective interferon c (IFNc)-mediated immune response. We tested the role of the JAK/ STAT1 signaling pathway in Leishmania pathogenesis by utilizing knockout mice lacking the signal transducer and activator of transcription 1 (Stat1) and derived macrophages. Unexpectedly, infection of Stat1-deficient macrophages in vitro with promastigotes from Leishmania major and attenuated LPG1 knockout mutants (lpg2) specifically lacking lipophosphoglycan (LPG) resulted in a twofold increased intracellular growth, which was independent of IFNc and associated with a substantial increase in phagosomal pH. Phagosomes in Stat12/2 macrophages showed normal maturation as judged by the accumulation of the lysosomal marker protein rab7, and provided normal vATPase activity, but were defective in the anion conductive pathway required for full vesicular acidification. Our results suggest a role of acidic pH in the control ofintracellular Leishmania growth early during infection and identify for the first time an unexpected role of Stat1 in natural anti-microbial resistance independent from its function as IFNc-induced signal transducer. This novel Stat1 function may have important implications to studies of other pathogens, as the acidic phagolysosomal pH plays an important role in antigen processing and the uncoating process of many viruses.