1. The oxygen luminometer is a sensitive device for the continuous measurement of small amounts of oxygen. An inert carrier gas flows at a controlled rate through an electrolysis cell, then through a sample cuvette, and finally into a solution of luminol dianion in dimethyl sulfoxide. The light generated by the chemiluminescent reaction of the luminol dianion with the oxygen is measured with a photomultiplier. Calibration, either internally with the water electrolysis cell or externally with premixed gases, shows the response of the system to be linear over a range of greater than four orders of magnitude in oxygen concentration. The sensitivity corresponds to a rate of oxygen production of 10 −13 mole/sec or to a change in oxygen concentration of 0.01 ppm in the gas or of 10 −11 M in water at room temperature. The device has a response time of about 1 min and the output is a direct measure of the rate of oxygen production in absolute units. 2. The oxygen luminometer is suitable for measuring oxygen production in photosynthetic systems. A mutant of Chlorella which requires light for chlorophyll synthesis is shown to lose its chlorophyll and its oxygen-producing activity in the dark by simple dilution on cell division.