The on-line estimation method of an "inherent" respiratory quotient (RQ(inh)) from a conventional "apparent" one (RQ(app)) based on exit-gas analysis was proposed. The value of RQ(inh) is a reasonable indicator for fermentation under alkaline conditions where the difference between Re,RQ(inh), and R& PP can not be negligible. The theoretical reassessment of the relation between RQ(inh) and RQ(app) was made taking account of the direct reaction of CO2 and OH- in liquid phase. The practical usefulness of the estimated RQ(inh) value was verified in the fermentation of aalkaliphilic Bacillus sp. employed for enzyme production on an industrial scale.