Tagetes species (marigolds) form thiophenes, heterocyclic sulphur-containing compounds with a strong nematicidal-activity. The highest thiophene concentrations are found in the roots. In in-vitro plant-cell cultures, thiophene accumulation increases 100-fold when root formation is initiated. This led us to hypothesize that the root-inducing phytohormone auxin is a factor regulating both thiophene biosynthesis and rhizogenesis. Roots transformed by Agrobacterium rhizogenes displayed an increased sensitivity to indoleacetic acid (IAA). In these roots, thiophenes were labelled by adding [35S]-sulphate to the culture medium. These experiments enabled us to quantify thiophene synthesis and degradation.