A method for on-line estimation of the fermentation parameters (substrate, product, and cells) of an alkaliphilic Bacillus sp. by utilizing data on the O-2 and CO2 tensions in the exhaust gas and the pH of the fermentation broth was investigated. The estimation was based on the stoichiometric relations concerning the weight of CO2 formed, O-2 consumed, product formed, and substrate consumed by the bacterium and on the ratio of the mass of the product to that of the cell biomass. During the fermentation, in which the pH range was 7.8-8.8, the parameters were successfully estimated by considering the reactions of CO2 to produce HCO3- and/or CO32- in the liquid-phase as well as the desorption of CO2 to the aeration gas. It was verified that the proposed estimation method is applicable to industrial-scale fermentation using an alkaliphilic cellulase-producing Bacillus sp. The correlation coefficients between the estimated and observed fermentation parameters exceeded 0.990, indicating that the method is capable of evaluating fermentation parameters with an accuracy satisfactory for industrial use.