Fluorescence kinetics were measured in membranes of a photosystem II-deletion mutant of the cyanobacterium Synechocystis sp. PCC 6803 containing photosystem I as the only reaction center and of the anoxygenic photosynthetic bacterium Heliobacillus mobilis. The measurements were performed under conditions where forward electron transfer to secondary acceptors was inhibited by the strong reductant sodium dithionite. Delayed fluorescence due to recombination of the primary radical pair P+A0- in both species was found to consist of several kinetic components. The longest-lived component had a lifetime of 35 ns in photosystem I and 18 ns in H. mobilis, respectively, which corresponds with the lifetime of the primary radical pair. Delayed fluorescence components with lifetimes of about 4 ns, 1 ns, and 200 ps were also observed in both species and were attributed to relaxations within the radical pair. A standard free energy difference of 0.18 eV was calculated for both species between the relaxed primary radical pair and the excited antenna at room temperature. A value of 0.25 eV was estimated for the free energy difference between the relaxed primary radical pair and the excited primary donor. The temperature dependence of the delayed fluorescence between 25 and 2 degrees C indicated that more than half of the free energy difference is due to enthalpy. Our measurements indicate an overall similarity between the primary electron transfer process in the Fe-S type (or low potential) reaction centers and the (bacterio)pheophytin-quinone type (or high potential) reaction centers found in purple photosynthetic bacteria and photosystem II.