An instrument to measure fluorescence lifetimes of the order of millimicroseconds is described. It utilizes a hydrogen flash lamp and high‐speed oscilloscope. The fluorescence decay is displayed on the oscilloscope, and using the method of moments the observed fluorescence is corrected for the response of the lamp and the detection equipment. The instrument was used to study the primary photochemical processes of photosynthesis. Measured lifetimes are given for the various photosynthetic pigments (in vitro), some organic dyes, and chlorophyll fluorescence in algae. With this instrument it has been possible to measure the delayed energy transfer in living systems.