A method for studying microbial respiration of low oxygen concentrations (10(-5)--10(-7) M) is described. The method was used for examining the respiration chain of the bacterium Escherichia coli and the yeast Saccharomyces cerevisiae. The following constants of the respiration chain for oxygen were calculated using the parameters of delayed bacterial fluorescence: the rate of respiration, the Michaelis constant, the concentration of oxygen critical for respiration. The decrease in the rate of respiration of low oxygen concentrations consists of two phases; this is related to the existence of two different pathways of oxidation in the terminal region of the respiration chain.