The photoexcited triplet states of bacteriochlorophyll a, ³BChl a, and of the primary donor in reaction centers of Rhodobacter sphaeroides R-26, ³P865, are investigated by pulsed EPR and ENDOR spectroscopy. In ³P865 a splitting of ENDOR lines and reduction of corresponding positive and negative hyperfine couplings as compared with the monomeric ³BChl a is observed. This indicates an asymmetric distribution of the triplet excitation over the two BChl a moieties, PL and Pm, forming ³P865. Based on the signs of the hyperfine couplings and on a comparison with the cation and anion radical of BChl a an assignment to nuclei in the different dimer halves is proposed. This yields an estimate for the extent of delocalization of the triplet excitation over PL and Pm and for the charge transfer contribution of ³P865.