The intracellular pH (pHi) of a,dicarboxylic acid producing Candida tropicalis was determined by a fluorescence technique using the pH-sensitive fluorescent probe 5(6)-carboxyfluorescein diacetate. Fermentations with n-tridecane substrate to produce a,-tridecanedioic acid were carried out in a 5-l bioreactor in which growth and production were separated. During the growth phase, the measured pHi values were varied from 5.65 to 6.15 in all the experiments performed under different constant pH-operating conditions. The specific rates of growth, sucrose consumption, CO2 production, and O2 consumption were correlated with pHi. Cytochrome P450 monooxygenase (P450), which catalyzes n-alkane hydroxylation, was only slightly expressed during the growth phase. During the first 6h of the production phase, P450 activity was induced rapidly accompanying higher pHi. A much higher level of P450 activity was observed at pHi of 6.55