Cercospora cruentaSacc, the causal organism of a leaf spot ofVigna catjangEndl., was investigated with respect to (1) its ability to synthesize thiamine, (2) the presence of thiamine in its mycelium, and (3) possible release of thiamine from the mycelium into the medium. The fungus was found to synthesize thiamine from a medium containing inorganic salts and dextrose. The thiamine was found chiefly stored in the mycelium with only a trace in medium in which the fungus had grown for 50 days at 25 °C.