Factors affecting hyaluronic acid production by Streptococcus zooepidemicus UN-7 were studied. Two types of the factors which were divalent cations and the precursors or the intermediates in hyaluronic acid biosynthetic pathway were investigated. Hyaluronic acid production in the control experiment in shaken flask was about 620-700 mg.l[superscript -1]. When single factor was added, MnSO[subscript 4].H[subscript 2]O (10 mg.l[superscript -1]) gave the best yield with the maximum amount of 880 mg.l[superscript -1] whereas similar yields of about 825-830 mg.l[superscript -1] were obtained from both glutamine (125 mg.l[superscript -1]) and glucose-6-phosphate (5 micromolar) and the yield of about 800 mg.l[superscript -1] was obtained from pyruvate (15 mg.l[superscript -1]). Addition of MnSO[subscript 4].H[subscript 2]O in combination with each fo those compounds at the above concentrations resulted in the best yield of 950 mg.l[superscript -1] with pyruvate whereas glucose-6-phosphate gave the second best of 930 mg.l[superscript -1] but the other compounds had no effect in increasing production yield from that of with MnSO[subscript 4].H[subscript 2]O alone. Cultivation in a 5 L- fermentor at agitation speed of 300 rpm and aeration rate of 1.5 vvm, the maximum yield of 2,144 mg.l[superscript -1] was obtained at 30 h of cultivation from the control experiment. Addition of MnSO[subscript 4].H[subscript 2]O (10 mg.l[superscript -1]) yielded 2,215 mg.l[superscript -1] at hour 30 with maximum yield of 2,572 mg.l[superscript -1] at hour 39. Addition of pyruvate (15 mg.l[superscript -1]) gave maximum yield of 2,541 mg.l[superscript -1] at hour 30. Addition of MnSO[subscript 4].H[subscript 2]O in combination with pyruvate at the above concentrations yielded 2,342 mg.l[superscript -1] at hour 30 and gave maximum yield of 2,903 mg.l[superscript -1] at hour 48. When fermentation was performed under the same conditions except at the agitation speeds of 400 and 500 rpm, the yields of 2,670 and 2,624 mg.l[superscript -1] were obtained at hour 30, respectively and reached the maximum yields of 2,949 mg.l[superscript -1] at hour 45 for 400 rpm and 2,903 mg.l[superscript -1] at hour 33 for 500 rpm.