The effect of dissolved oxygen on citric acid production and oxygen uptake by Candida lipolytica Y 1095 was evaluated in cell recycle and fed-batch fermentation systems. The maximum observed volumetric productivity, which occurred at a dilution rate of 0.06 h(-1), a dissolved oxygen concentration of 80%, and a biomass concentration of 5% w/v, in the cell recycle system, was 1.32 g citric acid/L . h. At these same conditions, the citric acid yield was 0.65 g/g and the specific citric acid productivity was 24.9 mg citric acid/g cell h. In the cell recycle system, citric acid yields ranged from 0.45 to 0.72 g/g. Both the volumetric and specific citric acid productivities were dependent on the dilution rate and the concentration of dissolved oxygen in the fermenter. Similar productivities (1.29 g citric acid/L . h) were obtained in the fed-batch system operated at a cycle time of 36 h, a dissolved oxygen concentration of 80%, and 60 g total biomass. Citric acid yields in the fed-batch fermenter were consistently lower than those obtained in the cell recycle system and ranged from 0.40 to 0.59 g/g. Although citric acid yields in the fed-batch fermenter were lower than those obtained in the cell recycle system, higher citric:isocitric acid ratios were obtained in the fed-batch fermenter. As in the cell recycle system, both the volumetric and specific citric acid productivities in the fed-batch fermenter were dependent on the cycle time and dissolved oxygen concentration.