Batch and continuous fermentation of cheese whey to lactic acid were carried out in a fluidized bed reactor by using Lactobacillus bulgaricus immobilized in 3% (w/v)garose. Five batch fermentations conducted under uncontrolled pH conditions and initial lactose concentrations ranging from 40 to 60g/L gave an average final lactic acid concentration of 6·9 g/L in a 48 to 50 h period. Continuous fermentation under controlled pH conditions at four different dilution rates ranging from 0·043 to 0·428 h^-1 and a lactose inlet concentration of 43·5 g/L resulted in a lactic acid yield of 0·26mg/mg lactose consumed. The maximum lactic acid concentration was 10·0 mg/mL at a dilution rate of 0·086 h^-1 with a substrate utilization of 90·5∥. The mathematical model of the system for batch and continuous operations was developed and the model parameters were determined experimentally and by non-linear multiple regression analysis.