The growth of 24 Aspergillus isolates at low oxygen tensions was assessed. Isolates selected included A. fumigatus (10), A. terreus (6), A. niger (6), A. nidulans (1) and A. flavus (1). Three different agar media were used--potato dextrose agar (PDA), pH 5.6; brain heart infusion (BHI), pH 7.4; and a specially developed medium (Hall's) containing resazurin--with oxygen concentrations of 0, 0.025, 0.1, 0.5 and 2.5%. The CO2 concentration was 5%. Agar plates were inoculated with 2 x 10(7) conidia/ml, loaded into jars and flushed with a special gas mixture at 37 degrees C. The plates were inspected at intervals of 3, 5 and 10 days. On Hall's medium, none of the isolates grew at oxygen concentrations of 0 or 0.025%, but 21 (88%) of 24 grew at 0.1%. On PDA and BH1, all 14 isolates tested grew at oxygen concentrations of 0.5 and 2.5%. Three of these 14 conidiated on PDA at oxygen 0.5% and 12 of 14 conidiated on PDA at oxygen 2.5%. None grew without oxygen on these media. Thus, pathogenic Aspergillus spp. are capable of growth at low oxygen tensions, and this may have implications for pathogenicity and antifungal activity.