Photosynthetic reaction centers (RCs) fromRhodobacter sphaeroides capture solar energy by electron transfer from primary donor D to quinone acceptor QB through the active A branch of electron acceptors. The light-induced electron paramagnetic resonance (EPR) spectrum from native RCs that had Fe2+ replaced by Zn2+ was investigated at cryogenic temperature (80 K, 35 GHz). In addition to the light-induced signal due to the formation of D+.Q A −. observed previously, a small fraction (ca. 5%) of the signal displayed very different characteristics. The signal was absent in RCs in which the QB was displaced by the inhibitor stigmatellin. Its decay time (τ=6 s) was the same as observed for D+.Q B −. in mutant RCs lacking QA, which is significantly slower than for D+.Q A −. (τ=30 ms). Its EPR spectrum was identical to that of D+.Q B −. . The quantum efficiency for forming the major component of the signal was the same as that found for mutant RCs lacking QA (ϕ=0.2%) and was temperature independent. These results are explained by direct photochemical reduction of QB via B branch electron transfer in a small fraction of native RCs.