Abstract: A total of 14 isolates of Aspergillus niger were obtained from soil samples of two different locations in Saudi Arabia. These cultures were screened for their ability to produce citric acid in date syrup medium under free and immobilized cells systems. Maximum productivity was about 1.44 times higher in the immobilized system compared to free cells. A. niger j4 isolated from Jazan region was selected as a good producer for optimization of the citric acid fermentation in immobilized cells technique. Citric acid production was increased gradually and reached to maximum value (30.6 g L-1) after 6 days of cultivation. Optimum production was achieved at 15% sugar concentration with consumed sugar of 49.5%. The highest value of citric acid (42.5 g L-1) was obtained at pH 5.5 with increasing the consumed sugar to 61.7%. A positive relationship between citric acid production and incubation temperature was also observed up to 30°C.