The photosystem I (PSI) core complex of oxygenic photosynthesis is an integral pigment-protein complex that incorporates both the antenna and the reaction center (RC). It binds about 100 Chl alpha and 20 beta -carotene molecules. In the PSI core complex of the cyanobacterium Synechococcus elongatus, a total of about 9 antenna Chi a molecules are;ed shifted with respect to the primary electron donor, which absorbs at 700 nm. We have studied energy transfer and trapping processes in trimeric PSI complexes of this species at femtosecond resolution by means of the fluorescence-upconversion technique: By simultaneously analyzing the fluorescence upconversion results and those obtained with a streak camera with picosecond resolution and multichannel detection (Gobets, B.; et al, Biophys. J., in press), we have mapped out the energy transfer processes that follow immediately after photon absorption. Equilibration among Chi a pigments in the bulk antenna was found to occur with a time constant of 360 fs. A major energy equilibration phase between bulk Chi a and the red-shifted antenna Chls occurs in 3.6 s. A slow phase in energy equilibration takes place in 9.8 ps, after which the excitations are trapped by the RC in 38 pst Fluorescence anisotropy measurements indicated an initial anisotropy of 0.30, which decayed biphasically with a major fast phase of 160 fs and a minor slow phase of 1.8 ps to a final anisotropy of 0.06. The 160 fs phase is assigned to elementary energy transfer steps in the bulk Chi a antenna, and the 1.8 ps phase to further equilibration processes, possibly involving energy transfer to or among red-shifted Chls. Energy transfer from p-carotene to Chi a was found to proceed both from the S-2 State and the S-1 state, with the majority of transferred excitations (60%) originating from the S2 state, resulting in an estimated overall yield of similar to 90%. A comparison is made with the PSII core antenna protein CP47, which binds the same pigments but has a substantially lower carotenoid-Chl a energy transfer yield of similar to 35% (van Dorssen R. J.; et al. Biochim. Biophys. Acta 1987, 893, 267).