The depolarization of the bacteriochlorophyll fluorescence from the core light-harvesting antenna (LH-1) of Rhodobacter sphaeroides was time-resolved using fluorescence upconversion. Complexes isolated in the detergent octylglucoside and chromatophores of the LH-1-only mutant M2192 were studied. For both preparations, the anisotropy of the fluorescence drops from an initial value of similar to 0.4 to 0.07 with a biphasic decay characterized by time constants of approximately 110 and 400 fs. The decay of the anisotropy is modeled as energy transfer between bacteriochlorophyll dimers in the antenna. The results are discussed in relation with current models for the structural organization of the core antenna. Numerical modeling of the observed annihilation processes indicates a domain size of approximately 16 pigments (eight dimers) for detergent-isolated LH-1 complexes. Simulations show that the depolarization data are consistent with either a symmetric ring structure and an inhomogeneous spectral distribution function for the pigments or a ring made up of pigment clusters. Oscillations in the isotropic fluorescence with a 105 cm(-1) frequency suggest wave-packet motion in the excited state chromophore in analogy with the reaction center special pair, for which a similar frequency is observed in time domain experiments. These oscillations are supporting evidence for the hypothesis that the LH-1 antenna is built of bacteriochlorophyll dimers. The dephasing of the oscillations is slower than the observed depolarization time scale.