The photosynthetic oxidation of water to molecular oxygen is energetically driven by light-induced charge separations in the reaction center of photosystem II (PS II). The reaction is catalyzed by a tetranuclear manganese cluster contained in the oxygen-evolving complex (OEC). The OEC cycles through five different redox states termed S0 to S4, with S1 being the darkstable state. Oxygen is released during the S4 f S0 transition.1 The removal of one electron from the OEC on each S state transition leads to the idea that alternate S states should be paramagnetic because of their odd-electron number. The multiline EPR signal, which is the hallmark of the S2 state, establishes the odd-electron character of the Mn cluster in S2.2 The S1 state, one-electron reduced from S2, is paramagnetic but of even electron number, and a non-Kramers EPR signal is observed in parallel-polarized EPR.3 Because the S0 state is reduced by one further electron, it is expected to be an oddelectron or Kramers state observable with conventional EPR. Hence, it was somewhat surprising that no EPR signal had been reported for this state. This problem was recently resolved by Messinger et al. who observed a new EPR multiline signal in an S0* state, an S0-like state produced by reduction of the S1 state by hydroxylamine or hydrazine.4 The essential ingredient was the addition of 1.5% methanol. We now report the observation of this EPR signal in a physiological S0 state produced by three-flash illumination of dark-adapted PS II membranes. This EPR signal is sufficiently similar to that produced by NH2OH treatment so that, from the perspective of EPR, one need no longer distinguish the states prepared by the two methods. Furthermore, we describe a broad EPR signal for the S0 state in absence of methanol.