The thesis focuses on comparing Aspergillus niger ATCC 1015 and NRRL 2270 Strains. This is achieved by studying three engineered strains to know the exact mutation that results in the good yield and stress tolerance of A. niger NRRL 2270. The strains were fermented in the laboratory to study the citric acid yield, D-Glucose utilization, dry cell weight, and morphology. The fermentation was performed under controlled fermentation parameters. The thesis studied those strains to achieve a high citric acid yield because of its importance in industry. The thesis identified the best strains in productivity and molar yield.