Halobacterium salinarium, an extremely halophilic archae, is capable of consuming up to 95% of phosphate from culture medium. For growth, H. salinarium needs high concentrations of phosphate in the medium. At P-i concentrations of 0.05, 2.3 and 11.5 mM the maximum amounts of biomass were 2, 5 and 12 g/l, respectively. Most P-i was present in biomass as magnesium ortho-phosphate, the amount of which by the early stationary phase may reach near 90% from phosphate consumed by the cells. H. salinarium contains inorganic polyphosphate (polyP) in amounts comparable with those in eubacteria and yeast. However, the phosphorus of polyP was not more than 10% of all the phosphorus accumulated in the culture. H. salinarium is able to use the reserves of phosphorus both in the form of ortho-phosphate and in the form of polyP for growth on medium at minimum phosphate concentration (0.05 mM). In contrast with eubacteria and yeast the main reserve of phosphate in this archae is magnesium ortho-phosphate. (C) 2002 Elsevier Science Ltd. All rights reserved.