A flavinogenic mutant was derived from Candida boidinii by mutagenesis. The mutant was smaller than the wild type, did not grow on a minimal medium, and required L-tryptophan, L-leucine, inositol, and nicotinate for growth. The mutant was defective in the oxidative pentose phosphate pathway, lacking glucosed-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. The specific activities of the transaldolase and transketolase of the mutant were higher than those of the wild type. These high activities might direct the flux of the carbon source to the nonoxidative pathway with formation of a large amount of pentose phosphates, Increasing riboflavin synthesis. Under microaerobic conditions at 25 degrees C, 90 mg/l riboflavin was obtained.