This thesis deals with the reduction of mycotoxin contamination level during soybean fermentation (Thua-Nao). Beside this work, isolation, characterization, and ochratoxin A production ability of toxigenic fungi from French grapes were also study. Results of this latter part showed that Aspergillus carbonarius and Aspergillus niger are the most ochratoxin A producer in wine grape from France. Furthermore, Aspergillus japonicus can produce a little bit quantity of ochratoxin A in wine grape too. Regarding to the main part of the work, 23 isolates of Bacillus spp. were isolated from Thai Thua-Nao. An Aspergillus flavus aflatoxin producing strain was also isolated from Thua-Nao whereas an Aspergillus westerdijkiae was chosen as an OTA producing reference strain. The objectives were to find an efficient Bacillus strain for: Growth inhibition of Aspergillus flavus and Aspergillus westerdijkiae NRRL 3174. - Limitation of aflatoxin B1 production. ; - Mycotoxins, aflatoxin B1 and ochratoxin A detoxification. Among the results, Bacillus CM 21, which was identified later by ITS sequencing as Bacillus licheniformis, showed the highest ability on inhibition of growth of both Aspergillus strains and both of mycotoxins removal (decrease of 74% of AFB1 and 92.5% of OTA). Another Bacillus strain, MHS 13, inhibiting both Aspergillus growth and detoxifying 85% of AFB1 was identified as Bacillus subtilis. Finally, culture supernatant and cellular extract from both interested Bacillus strains were tested for aflatoxin B1 and ochratoxin A degradation ability in order to know their degradation mechanisms. Moreover, study on optimal condition for aflatoxin B1 and ochratoxin A degradation were also conducted. All results indicated that OTA was significantly degraded by culture supernatant from Bacillus licheniformis CM 21 (p lower than 0.0001) in OTalpha. The percentage of OTA degradation was 97.5% and the optimal activity of its culture supernatant was found at pH 7.0 and 37°C with 24 h culture incubation time and 2 h contact time. Moreover, OTA was also significantly degraded by culture supernatant from Bacillus subtilis MHS 13 (p lower than 0.0017) at pH 5.0 and 37°C with 48 h culture incubation time and 2 h contact time. The proposed degradation mechanism should be extracellular and carboxypeptidase A probably responsible for this degradation since no activity was found for the intracellular extract. However, AFB1 could be degraded by neither culture supernatant nor cellular extract from both of these microorganisms. Hence, the AFB1 detoxification mechanism may be due to non-enzymatic mechanism
