Lactococcus lactis IO-1 (JCM 7638) produced less than 10 g . l(-1) of acetate as a by-product from xylose in batch culture. This low level of acetate did not markedly inhibit growth or L-lactate production, while L-lactate an the other hand exhibited a strong inhibitory effect. An examination of the kinetics of lactate fermentation by strain IO-1 in xylose medium revealed that L-lactate was responsible for end-product inhibition of the non-competitive type. Numerical analysis of the results of fermentation resulted in the determination of a variety of kinetic parameters. Using these parameters, a computer simulation program developed in a previous study was modified in order to fit batch culture for xylose medium. The results from the computer simulation and fermentation experiments, which had the same initial conditions, were in good agreement.