Thiophenes are polyacetylene-related heterocyclic metabolites. Some of these compounds are phototoxic, but the bithiophenes occurring inTagetes mainly accumulate in the root where photo-activation is not likely to occur. A cell-free extract from the fungusFusarium oxysporum induced biosynthesis of hydrophilic thiophenes in root cultures and roots of seedlings ofTagetes patula. The thiophenes formed were partially excreted into the culture medium. The excreted thiophenes inhibited fungal growth in the absence of light and thus may play a role in the biochemical defense against pathogens.