In bacterial photosynthetic reaction centers, ultrafast singlet excited state energy transfer occurs from the monomeric bacteriochlorophylls, B, and bacteriopheophytins, I-I, to the homodimer special pair, a pair of strongly interacting bacteriochlorophylls. In the M202HL mutant, one of the bacteriochlorophylls comprising the special pair is replaced by a bacteriopheophytin, and this is called the heterodimer special pair or D. We report the direct observation of spontaneous fluorescence from B-1 in the heterodimer mutant. In contrast to results for the homodimer special pair where 1B decays with a rate constant of (similar to 160 fs)(-1) (King, B. A.; McAnaney, T. B.; de Winter, A.; Boxer, S. G. J. Phys. Chem. B 2000, 104, 8895-8902), B-1 decay in M202HL exhibits two components with rate constants (similar to 700 fs)(-1) and (similar to 190 fs)(-1); these are similar to what we reported earlier for the rise of D-1 spontaneous fluorescence (King, B. A.; Stanley, R. J.; Boxer, S. G. J. Phys. Chem. B 1997, 101, 3644-3648). In the double mutant M202HL/M182HL, where the accessory bacteriochlorophyll on the M side is replaced by a bacteriopheophytin, the absorption bands corresponding to the chromophores in the B-L and B-M binding sites are quite well resolved, and it is possible to preferentially excite the chromophore on either the L or the M side. Analysis of the rise of D-1 fluorescence in the double mutant supports the earlier assignment of the slower similar to 700 fs energy transfer component to B-1(L) --> D, while the faster similar to 190 fs energy transfer component is assigned to B-1(M) --> D. Replacement of bacteriochlorophyll by bacteriopheophytin in the BM binding site does not alter the time constants of the two energy transfer pathways. Excited state energy transfer to D is the same in Q(A)-depleted and Q(A)-reduced reaction centers, suggesting that electron transfer processes that be might sensitive to a charge on Q(A), such as B-1(L) --> BL+HL- do not compete with relatively slow B-1(L) --> D energy transfer. The results support earlier findings that singlet energy transfer from the monomeric chromophores along the L and M branches to the heterodimer special pair is asymmetric and is faster along the M side, in contrast to the homodimer special pair in wild type where the energy transfer rates along the two branches are-very similar. Thus, conversion of the special pair homodimer to a heterodimer breaks the symmetry of ultrafast energy transfer along the two branches of chromophores. These findings may provide information on differences in the electronic interactions on the L vs M sides of the RC that is relevant to unidirectional electron transfer.