Angle dependence of circular intensity differential scattering (CIDS) and of nonpolarized scattering was determined in isolated spinach chloroplasts at 514.5 nm. CIDS between 0° and 170° was independent of the nonpolarized scattering and showed intense lobes of alternating signs, exhibiting the negative and positive maxima around 15° and 70°, respectively. These results provide experimental evidence for the existence of large helically organized macroaggregates of pigment-protein complexes in thylakoid membranes. Modeling of the CIDS data by a simple helical array of uniaxial polarizable groups suggests that the chiral structure is left-handed with pitch and radius of the order of 385 nm. © 1988, American Chemical Society. All rights reserved.