Zearalenone (ZON) is a non-steroidal estrogenic mycotoxin produced by plant pathogenic species of Fusarium. As a consequence of infection with F. culmorum and F. graminearum, ZON can be found in cereals and derived food products. Several countries have established monitoring programs and guidelines for ZON levels in grain intended for human consumption and animal feed. We have developed a sensitive yeast bioassay allowing detection of the estrogenic activity of ZON in cereal extracts without requiring further clean up steps. The high sensitivity makes this assay suitable for low cost monitoring of contamination of small grain cereals with estrogenic Fusarium mycotoxins, but also attractive as a tool for basic research. We have successfully used yeast indicator strains to screen for mutants of F. graminearum which no longer produce detectable amounts of ZON, and have identified a plant cDNA encoding a ZON detoxification enzyme.