Isolated chloroplast membranes of the unicellular photosynthetic alga Chlamydomonas reinhardtii were cryofixed, freeze-fractured, and surface replicas observed by transmission electron microscopy. With the ac-31 mutant, and the in vitro conditions used, thylakoid membranes form stacked regions with fracture faces showing the characteristic particle sizes and distributions. See Fig 20 in Goodenough and Staehelin 1971 J Cell Biol 48:595-619.