Changes in the cell ultrastructure were studied during incubation of the obligate phototrophic blue-green alga Anabaena variabilis in the dark for a long time. The cells lost viability though their cell wall and cytoplasmic membrane were preserved; however, certain regions of the mureine layer were thickened in some cells. The membranes of the photosynthetic apparatus separate forming intrathylakoid spaces and the cytoplasm density decreases revealing phycobilisomes. During incubation in the dark for a long time, polyglucoside alpha-granules in the cytoplasm disappear, polyhedral bodies are preserved, and numerous large granules of average electron density and unknown nature appear. Transfer of the culture from the dark to the light when the cells are losing their viability results in intensive destruction of the thylakoids. Loss of viability by the cells of Anabaena variabilis in the dark is supposed to be due to irreversible damages of the membranes of the photosynthetic apparatus in these conditions.