Kinetic data are needed to develop basic understanding of fermentation processes and to permit rational design of continuous fermentation processes. The kinetics of the fermentation of glucose to lactic acid have been studied at six constant pH levels between 4.5 and 6.0 by measuring the instantaneous rates of bacterial growth and of lactic acid formation throughout each fermentation. It was found that the instantaneous rate of acid formation dP/dt, should be related to the instantaneous rate of bacterial growth dN/dt, and to the bacterial density N, throughout a fermentation at a given pH, by the expression dP/dt = alpha dN/dt + beta N when the constants alpha and beta are determined by the pH of the fermentation.