AbstractThe ability of a number of Ustilago species, especiallyUstilago maydis(DC.) Cda., to produce lysine and threonine was investigated. The organisms were grown in shake flasks or in 10‐l. fermentors. Lysine and threonine were found to be excreted into the medium both in the free and bound form. The bound amino acids could be released by acid hydrolysis or by enzymes from autolyzed cells. The optimal conditions for the release by autolysis were, in the case ofUstilago maydis(DC.) Cda., pH 4.3 and 45°C. An enzyme that could liberate lysine from the bound form(s) occurring in the broth was extracted from cells ofUstilago maydis(DC.) Cda. It exhibited an apparent pH optimum near 4.0. The effect of pH and temperature during the growth phase on the yield of lysine and threonine was studied in 10‐l. fermentat