A theory of one-color two-photon excitation fluorescence depolarization in organized molecular assemblies and solutions, is outlined and discussed. The discussed theory is based on the formalism of spherical irreducible tensorial sets and the group theory methods. The obtained general formulas are symmetry simplified and discussed for the case of macroscopically ordered planar membranes, Langmuir-Blodgett (LB) films, stretched polymer films, membrane vesicles, labeled macromolecules and solutions. (C) 2003 Elsevier Science B.V. All rights reserved.