  The existence of a light dark force mediator has been tested with the KLOE
detector at DAPHNE. This particle, called U, is searched for using the decay
chain phi-->etaU, eta-->pi+pi-pi0, U-->e+e-. No evidence is found in 1.5 fb-1
of data. The resulting exclusion plot covers the mass range 5<M_U<470 MeV,
setting an upper limit on the ratio between the U boson coupling constant and
the fine structure constant, alpha'/alpha, of <= 2x10^-5 at 90% C.L. for
50<M_U<420 MeV.
