The objective of this study was to examine the effect of gaseous ozone (O3) on (a) germination, (b) mycelial growth, (c) aflatoxin B1 (AFB1) production by strains of Aspergillus flavus at different water activity (aw, 0.89~0.97=13.1%~24% moisture content) and 25 oC in vitro. In addition, taking Egyptian peanuts as a research object, shelled peanuts at 0.93 aw were inoculated with two concentrations of A. flavus conidia (103, 105 conidia/g) and exposed to gaseous O3 and then stored. In all cases, exposure was for 30 min at 6 L/min. Generally, >100 ppm O3 significantly inhibited conidial germination of A. flavus strains (EGP-B07; SRRC-G 1907) on a defined yeast sucrose medium within 48 hrs. However, exposure of growing colonies of A. flavus to O3 with up to 300 ppm, had no effect on subsequent colony extension. The same concentration significantly affected AFB1 production, but only at 0.89 aw. Populations of both A. flavus strains in stored shelled peanuts exposed at 0.93 aw were significantly decreased at 100~400 ppm O3. However, AFB1 production was only significantly reduced in the 400 ppm O3 treatments at both inoculum load levels. These results are discussed in the context of O3 gas use for controlling A. flavus populations and AFB1 in shelled peanuts post-harvest.