Abstract: The fungus Clonostachys rosea ‘IK726’ has proven to be effective in biologicalcontrol of a range of plant diseases. Among these are fusariose caused by Fusarium graminearumor Fusarium culmorum – pathogens known to produce mycotoxins such as zearalenone (ZEN)and deoxynivalenol (DON). It has been shown that the zhd101 gene in C. rosea encodeszearalenone lactonohydrolase (ZHD) which breaks down the Fusarium mycotoxin (ZEN) to lesstoxic derivates in vitro. However, it has not been determined previously whether ZENdetoxification plays a role in biocontrol of F. graminearum on plants. Here we show thatC. rosea, via detoxification, counteracts exposure to ZEN during biological control interactionswith F. graminearum on barley seedlings and that this is an important trait for the efficacy ofbiological control performance.