Halosarcia pergranulata (P. G. Wilson) subsp. pergranulata is a member of the Salicornioideae and is native to Australia. Salt tolerance in H. pergranulata subsp. pergranulata was assessed by growing plants for 83 d at seven NaCl concentrations from 10 to 800 mol m(-3). Shoot biomass was greatest for plants grown at 10 to 200 mol m(-3) NaCl, while at salinities of 300 mol m(-3) or higher it was inhibited. There was little increase in succulence in response to NaCl, and it even declined at the highest salinities. The K+:Na+ molar ratio in succulent shoot tissues decreased from 0.30:1 in plants grown at 10 mol m(-3) NaCl to 0.02: I in plants at 600 mol m(-3), due to a three-fold increase in tissue Na+ concentration and a five-fold decline in tissue K+. The osmotic potential of sap (pi(sap)) in the expanding shoot tissues remained -1.7 to -2.5 MPa below the pi of the root medium. Na+ plus Cl- contributed 56 to 80% of the pi(sap) in plants grown at 10 to 800 mol m(-3) NaCl. These Endings show that H. pergranulata subsp. pergranulata has a high degree of NaCl tolerance, and that studies on solute compartmentation at the cellular level are required to elucidate the mechanisms by which this species tolerates very low tissue K+:Na+ ratios. (C) 1999 Annals of Botany Company.