Picosecond time-resolved fluorescence emission from the pigments of intact photosynthetic systems and isolated pigment-protein fractions has been used to probe the mechanism of energy transfer and the organization of the pigments. The fluorescence kinetics of chlorophyll and the phycobilins of the red alga, Porphyridium cruentum, are governed by time-dependent kinetics, but the observed time dependence of the chlorophyll a fluorescence decay from dark-adapted Chlorella pyrenoidosa and spinach sub-chloroplast fractions is still open to conjecture. In contrast to the green plants containing only chlorophyll and carotenoids, Porphyridium shows distinct emission bands for each the pigments in the transfer sequence. The rate of energy transfer in vivo has the empirical form: dS/dt = -1/2S At-1/2, where S is the excited-state population of the donor pigment and A is the overall rate of energy transfer to the acceptor pigment. The kinetic analysis can describe closely the observed fluorescence risetimes and lifetimes of the photosynthetic pigments of Porphyridium. The extremely rapid rates of energy transfer, determined by this treatment, imply that exciton migration within each pigment bed of the phycobilisome is less extensive than in the chlorophyll-antenna systems. Changes in the fluorescence yield and decay kinetics of chlorophyll a and allophycocyanin in vivo can be induced at high excitation intensities by exciton-exciton annihilation.