The effect of dissolved oxygen tension (DOT) on microbial cultures in wastewater treatment processes has been a controversial subject for the last two decades. Expanded application of the pure oxygen activated sludge process has developed data that centered the controversy around two issues: the effect of DOT on culture stoichiometry and the effect of DOT on settling rates of biological floes. This work investigates the stoichiometric effect of DOT. A carbon balance method was developed to monitor the stoichiometry of a continuous culture by analyzing influent and effluent, gas, liquid, and solid streams for carbon. Over the DOT range studied (3-4 mg/l) there was no direct effect of DOT on the stoichiometry. High DOTS did affect the balance of predators and prey in the culture by inhibiting the ciliates. In this way DOT indirectly affected the stoichiometry. Carbon dioxide concentrations varied unavoidably in the aeration gas providing the opportunity to analyze its effect. From limited data it appears to have a direct effect on stoichiometry with increased carbon dioxide inhibiting respiration.