Aspergillus niger, strains CK1−16, the bacteria commonly used in industrial production of citric acid, were implanted with 3.10×10,14, to 2.58×10,15, ions/cm,2, of 15 keV N,+, ions . Suevival rate of the bacteria at different doses was investigated, and mutagenic effects of the microbe were studied. From the ion-implanted specimens, we were able to obtain three mutant strains that produce increased yield of citric acid. The 71 hours yield of Mutant Strain 4#−8−1incubated in cassava and corn flour media is 14% higher than the CK1-16 strain, while Mutant Strains 4#−8−7 and 4#−8−7, incubated in corn flour media are 15.9% and 17.0% higher than the CK1-16 strain, respectively. The results also show that the mutant strains have high genetic stability.