While investigating the requirement for phagosomal alkalinization in the host defense against pulmonary aspergillosis, we observed high morbidity of p47phox-/- mice infected with pH-insensitive Aspergillus nidulans mutants despite a paucity of fungal growth. Fatal infection also resulted from a normally avirulent p-aminobenzoate auxotroph. This demonstrates that p47 phox-/- murine immunity contributes significantly to A. nidulans lethality. These data have wider implications for microbial virulence studies with p47phox-/- mice. Copyright © 2005, American Society for Microbiology. All Rights Reserved.