Biomass production of Bifidobacterium pseudocatenulatum G4 in a milk-based medium was carried out in a 2- and 10-L stirred tank fermenters. The effects of impeller tip speed (0.28, 0.56, and 0.83 m/s) and pH control (6.0, 6.5, and 7.0) on the biomass production were investigated. The growth performance in the 2-L fermenter was significantly improved when the impeller tip speed was heId constant at 0.56 m/s and the pH was controlled at 6.5. These conditions yielded a maximum biomass of $1.687{ imes}10^9cfu/mL$, a maximum specific growth rate of $0.504 h^{-1}$, a biomass productivity of $9.240{ imes}10^7 cfu/mL{cdot}h$, and a biomass yield of $9.791{ imes}10^{10} cfu/g$ lactose. The consumption of milk lactose resulted in the accumulation of 7.353 g/L acetic acid and 6.515 g/L lactic acid, with an acetic:lactic ratio of 1.129. Scale-up of the fermentation process to a 10-L fermenter based on a constant impeller tip speed of 0.56 m/s yielded reproducible results with respect to biomass production and cell viability.