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Chroococcidiopsis sp. cells from hot desert rock (Timna National Park, Israel) were grown in modified BG-11 medium which was either N-free or which contained different concentrations of nitrogen, provided as nitrate. The life cycle, ultrastructure and physiological activities of the cyanobacterium were investigated over a period of 4 months. No alterations were detected in cells grown in medium containing 250 or 125 mu g N ml(-1). In contrast, cells grown in medium with 62 mu g N ml(-1) or lacking nitrogen occurred only as isolated viable forms, after 3 months and 1 month of growth, respectively. In these cells a decrease in the content of cellular chlorophyll and phycocyanin paralleled undetectable O-2 evolution and depressed O-2 uptake. In addition, such isolated cells were characterized by a multilayered envelope, whereas, in the cytoplasm, vesiculated thylakoids and glycogen granules were observed. N-limited and N-starved cells recovered their cellular organization and physiological activities upon N repletion. These ’survival’ cells had a spore-like form, and were functionally comparable to akinetes.