The cytochrome b(6)f complex of oxygenic photosynthesis contains a single chlorophyll a (Chl a) molecule whose function is presently unknown. The singlet excited state of the Chl a molecule is quenched by the surrounding protein matrix, and thus the Chl a molecule in the b(6)f complex may serve as an exceptionally sensitive probe of the protein structure. For the first time, singlet excited-state dynamics were measured in well-diffracting crystals using femtosecond time-resolved optical pump-probe methodology. Lifetimes of the Chl a molecule in crystals of the cytochrome b(6)f complex having different space groups were 3-6 times longer than those determined in detergent solutions of the b(6)f. The observed differences in excited state dynamics may arise from small (1 - 1.5 angstrom) changes in the local protein structure caused by crystal packing. The Chl a excited state lifetimes measured in the dissolved cytochrome b(6)f complexes from several different species are essentially the same, in spite of differences in the local amino acid sequences around the Chl a. This supports an earlier hypothesis that the short excited state lifetime of Chl a is critical for the function of the b(6)f complex.