The structural organization of cells of the Brc-1 mutant of the unicellular green alga Chlamydomonas reinhardtii grown in the light and in the dark was studied. The Brc-1 mutant carries the brc-1 mutation in the nuclear gene LTS3. In the light, all membrane structures in its cells are normally formed and well developed. In the dark, the Brc-1 mutant cells grow and divide well, but none of their cell membranes (plasma membrane, tonoplast, mitochondrial membranes, nuclear envelope membranes, chloroplast membranes, thylakoid membranes, and Golgi dictyosomal membranes) are detected by electron microscopy. It is shown that even a short-term (for 1–10 min) exposure of the Brc-1 mutant cells to light led to the restoration of all these membrane structures. Possible causes for the observed changes in the membrane structure of the Brc-1 mutant are discussed.