Cetecol has been known as a component of melanin in teliospores of the corn smut fungus Ustilago maydis. Its metabolic precursor has been assumed to be benzoic acid but it has not been proven yet. This study was carried out to verify the synthesis of benzoic acid and to chase its metabolic origin in U. maydis. For this aim, the catabolic process of phenylalanine was investigated by culturing the fungus in the complete medium containing L- ¹⁴C-phenylalanine and ¹⁴C-trans-cinnamic acid. We detected trans-cinnamic acid, benzoic acid, 4-hydroxybenzoic acid and hydroxybenzoic acid derivatives from the extracts of the fungus cells and cultural filtrates by thin layered chromatography analysis. We also observed that the fungus could completely catabolize L- ¹⁴C-phenylalanine and produce ¹⁴CO₂ in the air. Conclusively, this study provided an evidence that U. maydis could produce benzoic acid through catabolic process of phenylalanine.