Abstract— The use of an inexpensive pulsed laser diode (Hamamatsu picosecond light pulser PLP‐01) as the excitation source for a single photon timing spectrolluorimeter with microchannel plate photomultiplier detection was dem‐onstrated. The performance of the instrument was tested with two very short‐lived fluorescent dyes and two pho‐tosynthetic systems with wcll‐defined decay characteristics. Individual fluorescence decays were analyzed by modeling with a convolution of the instrument response function to a sum of exponential decay components. Accurate fluorcscence lifetimcs of the dyes cryptocyanine (55 ps in acetone and 83 ps in ethanol) and 1,1‘‐diethyl‐2,2′‐dicarbocyanine iodide (13 ps in acetone and 26 ps in ethanol) were obtained by analysis of the decay kinetics with a single exponential component. Fits to the fluorescence decay kinetics of isolated photosystem I particles and intact cyanobacterial cells required three and four decay components. respectively. The decay kinetics of the isolated photosystem I preparation were dominated (99%) by a very fast 9 ps lifetime, reflecting the preparation's small antenna size of approximately 30 chlorophyll a. The cyanobackria showed decay components of 35 ps, 160 ps, 400 ps and 1.95 ns similar to those described previously by Mullincaux and Holzwarth (Rinchim. Biophys. Acfa1098, 68–78, 1991). The performance of the pulsed laser diode as an excitation source for single photon timing is discussed in comparison with conventional sources of picosecond light pulses.