In this study a commercial fermented milk produced in Korea and a Lactobacillus strain used for the product (L. casei) were found to affect mold growth and inhibit aflatoxin production by Aspergillus parasiticus ATCC 15517. Aflatoxins were determined using an HPLC system that consisted of a $C_{18}$ column and a fluorescence detector. When the fermented milk was added to the yeast-extract broth the levels of aflatoxin $B_{1};and;G_{1}$ significantly decreased by 48.6~58.1% and 29.8~34.2%, respectively (p<O.05). The mold growth in this case was reduced by comparison with the control (p<O.05). The pH values of the culture broth in the fermented milk-containing group were observed to be significantly lower than those in control group (p<O.05). In the mixed culture experiments (L. casei and A. parasiticus), lower levels of aflatoxin $B_{1};and;G_{1}$ were found in comparison with the control (monoculture). L. casei was found to be very inhibitory to the growth of A. parasiticus for 5 days, but no significant difference of mycelial weight was observed between the mixed culture and control at the end of incubation. The pH values of the culture broth in mixed culture were observed to be significantly lower than those in monoculture (p<O.05). The inhibitory effects of the fermented milk may be mainly due to the metabolites of L. casei as well as the strain itself.