Asteromonas gracilis, a green wall-less halotolerant alga, grows on salt concentrations from 0.5 molar NaCl (seawater) to saturation (4.5 molar NaCI). The specific growth rate was maximal at concentrations between 0.5 and 2.5 molar and only graduaDly decreased above 2.5 molar. Photosynthetic oxygen evolution was maximal over a range of salinities around 2.5 molar and the photosynthesis to respiration ratio showed a maximum at 1.5 molar NaCI. The alga accumulates large amounts of intraceDlular glycerol in response to saline conditions. The glycerol content of the cells varied in direct proportion to the extracellular salt concentration, being about 50 and 400 picograms glycerol per cell in algae grown at 0.5 and 4.5 molar NaCI, respectively. In salt concentrations lower than 3.5 molar and at growth temperatures below 40 C, essentialy al the glycerol was intraceHular. Above 3.5 molar NaCI, about 25 per cent of the total glycerol leaked slowly from the cels to the medium. Treating the algae for several minutes at temperatures exceeding 47 C caused 50 per cent release of the internal glycerol. At 60 C, 100 per cent of the glycerol was released. When the extraceHular salt concentration was increased or decreased, the intracelular glycerol varied accordingly, reaching its new intracelular level after a few hours. Both photosynthesis and respiration were inhibited on transfer of the cels from 1.5 to 3.5 molar NaCI but were not inhibited on transfer of the cels from 3.5 to 1.5 molar NaCI. The kinetics of photosynthetic resumption preceded the kinetics of glycerol biosynthesis. The above results indicate the existence of osmotic regulations in Asteromonasgracilis via the accumulation of intracelular glycerol.Asteromonas gracilis Artari is a green wall-less unicellular flagellate of the class Prasinophyceae present in salt marshes and small brine ponds (10,15,17,19,20). The algae are often subjected to widely fluctuating salt concentrations and can tolerate a broad range of salt, from low salinity to saturated NaCl solutions. In contrast to the many studies which were undertaken on the halotolerant alga Dunaliella (9) to elucidate the mechanism of its osmotic adaptation under salt stress (2, 4-8), the information on osmoregulation in Asteromonas is rather limited. Recently BenAmotz and Avron (5) showed that Asteromonas and Dunaliella produce and accumulate glycerol to a concentration isoosmotic with the medium and, thus, are able to survive in saline water. The communication presented here describes an investigation of the physiological behavior of A. gracilis under salt stress.