Filtrates from certain liquid cultures ofChlorella vulgaris(Columbian strain) developed sufficient peroxide content, after a few days’ storage, to inhibit further growth ofChlorellawhen they were used to prepare fresh culture media. Similar inhibition was observed whenChlorellawas cultured in media containing various added organic peroxides. Growth ofChlorellawas not limited by autoinhibitor production under the conditions employed, and no peroxide or other inhibitor was detected in solvent extracts of the cells or fresh filtrates.