PhotobacteriumphosphoreumIFO13896 emits light strongly when cultured in medium containing 3% NaCl, but only weakly in medium containing 1% is known that dim or dark mutants appear frequently and spontaneously from this parent confirm that riboflavin biosyn-thesis is stimulatedwhen theluxoperon is active, the amount of light emitted and flavins synthesized under strongly or weakly light emitting conditions was determined.