The effect of an external electric field on fluorescence quenching due to electron transfer from a photoexcited electron donor to an acceptor has been analyzed theoretically. The model predicts that at weak fields the variation DeltaI(c,F)/I(c,0) in the steady-state monomer fluorescence intensity induced by an external electric field is proportional to the square of the field strength F and to the concentration of acceptors c. Similar relations have been reported for the fluorescence intensity of ethylcarbazole doped in poly-methyl-methacrylate films in the presence of dimethyl terephtathalate and an external electric field with a strength up to 0.01 V/Angstrom. The effect of the free energy change of the electron transfer reaction on the c and F dependencies of DeltaI(c,F)/I(c,0) has been discussed within the framework of the present model.