In this paper we report the results of measurements performed by FLIM on the photoreceptor of Euglena gracilis. This organelle consists of optically bistable proteins, characterized by two thermally stable isomeric forms: A(498). non fluorescent and B-462, fluorescent. Our data indicate that the primary photoevent of Euglena photoreception upon photon absorption consists of two contemporaneous different phenomena: an intramolecular photo-switch (i.e., A(498) becomes B-462), and a intermolecular and unidirectional Forster-type energy transfer. During the FRET Process, the fluorescent B-462 form acts as donor for the non-fluorescent A(498) form of the Protein nearby, Which acts as acceptor. We hypothesize that in nature these phenomena follow each other with a domino progression along the orderly organized and closely packed proteins in the photoreceptor layer(s), modulating the isomeric composition of the photoreceptive protein pool. This mechanism guarantees that few photons are sufficient to produce a signal detectable by the cell. (C) 2009 Elsevier Inc. All rights reserved.