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Orientation of lipophilic dye molecules within a biological membrane can report on the molecular environment, i.e., the physical and chemical properties of the surrounding membrane. This fact, however, remains under-utilized, largely because of our limited quantitative knowledge of molecular orientational distributions and the fact that robust techniques allowing experimental observation of molecular orientations of dyes in biological membranes are only being developed. In order to begin filling this lack of knowledge and to develop appropriate tools, we have investigated the membrane orientational distribution of the 3-hydroxyflavone-based membrane dye F2N12S. Results of our single- and two-photon polarization microscopy observations of linear dichroism of F2N12S-labeled giant unilamellar vesicles are consistent with a Gaussian-like orientational distribution of the transition dipole moment of the dye, with a …