Fast time-resolved EPR spectroscopy is used to study electron spin polarization (ESP) in perdeuterated native, Fe2+-containing reaction centers (RCs) of photosynthetic purple bacteria. The spin-correlated radical pair-(SCRP) model (previously used to simulate ESP observed in Fe-depleted RCs (Here, P. J.; Hunter, D. A.; McKie, C. D.; Hoff, A. J. Chem. Phys. Lett. 1987, 137, 495) is extended to include the large anisotropy arising from the magnetic interactions between Fe2+ and the reduced primary electron-acceptor quinone (Q(A)(.-)), which results in different quantization axes for the P-.- and the (Q(A)(.-)Fe(2+)) spins. Using spectral simulations, it is shown that the ESP spectrum is solely due to the P-.+ part of the spin-correlated radical pair [P-.+(Q(A)(.-)Fe(2+))], whereas the rapid decay of the spin-polarized signal is due to spin-lattice relaxation of the (Q(A)(.-)Fe(2+)) complex. The simulations are very sensitive to the relative orientation of the g matrices of P-.+ and (Q(A)(.-)Fe(2+)). Using orientation II of the g matrix of the oxidized primary donor P-.+ (Klette, R.; Torring, J. T.; Plate, M.; Mobius, K.; Bonigk, B.; Lubitz, W. J. Phys. Chem. 1993, 97, 2015), the orientation of the g matrix of (Q(A)(.-)Fe(2+)) is assessed. Finally, it is shown that the ESP spectrum of perdeuterated native, Fe2+-containing RCs of Rhodopseudomonas (Rps.) viridis is virtually identical to the spectrum obtained for perdeuterated native Rhodobacter (Rb.) sphaeroides, showing an AEA pattern (A denotes absorption and E emission). This result indicates that the magnetic axes of P-.+ and (Q(A)(.-)Fe(2+)) have (nearly) the same directions relative to one another in both species.