1. The addition of aniline to cultures of several yeasts, Fusarium oxysporum and Neurospora crassa growing with protocatechuate as sole carbon source resulted in the precipitation of dianilino-o-benzoquinone (anil). This product was also formed, however, if the medium was uninoculated. 2. The physical presence of yeast cells (living or dead) increased the anil yields in Debaryomyces subglobosus cultures. 3. No anil was formed if p-hydroxybenzoate was the growth substrate. 4. o-Benzoquinone was a strong inhibitor of protocatechuate 3,4-oxygenase and catechol 1,2-oxygenase in these fungi. 5. It was concluded that o-benzoquinone formation from protocatechuate is independent of living yeast.