Mycotoxin contamination of food and grains is a serious problem globally with feed stocks often being contaminated with multiple, different mycotoxins. Many studies have successfully isolated microorganisms able to degrade these compounds separately. It is unclear if the presence of other mycotoxins will interfere with the degradation efficiency of a particular microorganism or if a microorganism can simultaneously degrade a mixed substrate of mycotoxins. A study was conducted to determine the effect of mixed mycotoxin substrates on an aflatoxin-degrading microorganism, Rhodococcus corynebacterioides DSM 120151. The bacterium was tested in complex liquid medium containing either 5 mg/L of aflatoxin B1, 5 mg/L of zearalenone or the combination of the two mycotoxins at 5 mg/L each. Cultures were incubated at 28⁰C.  Samples were removed at 24, 48, and 72 hours and extracted with ethyl acetate. Mycotoxins in the extracted samples were quantified using high-performance liquid chromatography (HPLC). The addition of zearalenone had no significant effect on the degradation of aflatoxin B1 by Rhodococcus corynebacterioides DSM 120151. With and without the addition of zearalenone, the bacterium was capable of degrading 100% of aflatoxin B1 within 72 hours. However, zearalenone showed no degradation with or without aflatoxin B1 within 72 hours. This lack of effect on the degradation of aflatoxin B1 and inability to degrade zearalenone may indicate the specificity of the enzymes and genes responsible for degradation of aflatoxin B1.