The precise microecology of Blastomyces dermatitidis is unknown, but the fungus has been associated with nitrogenous waste products and rapidly changing environmental conditions. Ammonia accumulates in certain microenvironments, is toxic to most fungi, but may not be identified in processed soil samples. Ammonia tolerance of B. dermatitidis was investigated with two strains recovered in Wisconsin, one from a dog and the other from an environmental source. The samples were grown on phosphate and HEPES buffered agar media supplemented with mineral salts, low (1 g/l) and high (20 g/l) dextrose and increasing amounts of ammonium sulfate, at pH 7-8.2, in gas-impermeable bags at 20 degrees C. Moderate mold growth and sporulation of the strains were observed at low glucose concentration and calculated ammonia concentrations of 4.2 mmol/l when plates were inoculated with either mold or yeast forms. Three recent B. dermatitidis human clinical isolates also exhibited similar growth on this media, and 4/5 strains tolerated ammonia concentrations of 42-62 mmol/l. Growth of virtually all soil fungi from 206 aqueous slurries of fresh and frozen soil from the northern USA and Canada was inhibited at ammonia concentrations of 2.1-4.2 mmol/l. The ability of B. dermatitidis to survive and grow in organic carbon-poor, high ammonia microenvironments may be important to the competitive success of this fungus. These findings may have implications for other dimorphic fungi.