The physiological literature on Nitrobacter is reviewed and a list of the unsolved problems presented. Modifications to the action spectrum apparatus built by Brooks (1967) are described. The apparatus was then used with Nitrobacter agilis, ATCC no. 14123, to obtain an action spectrum of the relief of carbon monoxide inhibition by light. The results of this study indicate that cytochrome a₁ is active as a terminal oxidase. The possibility of other cytochromes, principally cytochrome ₀, acting as oxidases has not been proven or ruled out. The results of a study on the rate of oxygen uptake versus oxygen concentration are also reported; the Km(0₂) values range from 0.021 to 0.055 mM oxygen. The action spectrum reported here is the. first one to be determined on a chemolithotropic bacterium. / Science, Faculty of / Physics and Astronomy, Department of / Graduate