Toxoplasma gondii belongs to a group of highly virulent intracellular parasites that reside in host cell vacuoles which resist typical phagosome-lysosome fusion1. Live Toxoplasma replicate prodigiously within modified phagocytic vacuoles formed during invagination of the host plasma membrane2,3. In contrast, heat-killed Toxoplasma or specific antibody (heat-inactivated)-coated live Toxoplasma-containing vacuoles readily undergo lysosome fusion and digestion in normal macrophages2,4. Of newly recognized significance to Toxoplasma survival is the microbicidal effect of phagosome acidification, which reportedly can occur independently of fusion with other acidic vesicles5-8. We report here that modified live Toxoplasma-containing vacuoles fail to acidify in normal macrophages, as indicated by the sensitive pH probe fluorescein. In contrast, when live Toxoplasma are coated with specific antibody (heat-inactivated), they trigger phagosome acidification when entering normal macrophages. A similar acidification is observed when normal phagocytes ingest dead Toxoplasma. Extracellular Toxoplasma are highly susceptible to acidic pH conditions, indicating that the acidification block in the modified vacuoles may be important for intracellular survival.