The oxidation and growth kinetics of ferrous iron with Thiobacillus ferrooxidans in continuous cultures was examined at several total iron concentrations. On-line off-gas analyses of O2 and CO2 were used to measure the oxygen and carbon dioxide consumption rates in the culture. Off-line respiration measurements in a biological oxygen monitor (BOM) were used to measure directly the maximum specific oxygen consumption rate, q(O2,max), of cells grown in continuous culture. It was shown that these reproducibly measured values of q(O2,max) vary with the dilution rate. The biomass- specific oxygen consumption rate, q(O2), is dependent on the ratio of the ferric and ferrous iron concentrations in the culture. The oxidation kinetics was accurately described with a rate equation for competitive ferric iron inhibition, using the value of q(O2,max) measured in the BOM. Accordingly, only the kinetic constant K(s)/K(i) needed to be fitted from the measurements. A new method was introduced to determine the steady-state kinetics of a cell suspension in a batch culture that only takes a few hours. The batch culture was set up by terminating the feeding of a continuous culture at its steady state. The kinetic constant K(s)/K(i) determined in this batch culture agreed with the value determined in continuous cultures at various steady states.