The initial electron transfer in reaction centers from Rhodobacter sphaeroides R26 was studied by a subpicosecond transient pump-probe technique. The measured kinetics at various wavelengths were analyzed and compared with several mechanisms for electron transfer. An unambiguous determination of the initial electron transfer mechanism in reaction centers cannot be made by studying the anion absorption region (640-690 nm), due to the spectral congestion in this region. However, correlations between the stimulated emission decay of the excited state of the special pair, P*, at 926 nm and bleaching of the bacteriopheophytin Qx absorption at 545 nm suggest that the electron transfer at 283 K is dominated by a two-step sequential mechanism, whereas one-step superexchange and the two-step sequential mechanism have about equal contributions at 22 K.