The influence of high carbon dioxide and low oxygen concentrations on growth by the foodborne fungal species, Mucor plumbeus, Fusarium oxysporum, Byssochlamys fulva, Byssochlamys nivea, Penicillium commune, Penicillium roqueforti, Aspergillus flavus, Eurotium chevalieri and Xeromyces bisporus was investigated. Production of aflatoxin by A. flavus, patulin by B. nivea and roquefortine C by P. roqueforti was also studied. Fungal growth was evaluated under atmospheres consisting of 20, 40 and 60% CO(2) plus <0.5% O(2), on two media, Czapek Yeast Extract agar and Potato Dextrose agar. Several methods for measuring fungal growth were used: colony diameter, ergosterol content, hyphal length and/or mycelium dry weight. Among the nine species, three groups were distinguished with respect to their growth responses under modified atmospheres: (i) species which did not grow in 20% CO(2) <0.5% O(2) (P. commune, E. chevalieri and X. bisporus); (ii) species which grew in 20% CO(2) <0.5% O(2), but not 40% CO(2) <0.5% O(2) (P. roqueforti and A. flavus); (iii) species which grew in 20%, 40% and 60% CO(2) <0.5% O(2) (M. plumbeus, F. oxysporum, B. fulva and B. nivea). Facultatively anaerobic behaviour was observed in these last four species, which grew under the same conditions as the obligate anaerobe, Clostridium sporogenes. The production of aflatoxin, patulin, and roquefortine C was greatly reduced under all of the atmospheres tested.