The tertiary sulphonium compound, β-dimethylsulphoniopropionate (DMSP) and the quaternary ammonium compounds glycine betaine and homarine are important osmotica in Platymonas subcordiformis cells. Following hypersaline stresses the compounds were accumulated after a lag period of 3 h and equilibrium concentrations were reached 6 h later. In contrast to these organic solutes, mannitol was synthesised immediately and equilibrium concentrations were reached within 90 min. Hyposaline stresses induced losses of the organic solutes from the cells. The ions K(+), Na(+), Cl(-) and the above organic solutes can account for the osmotic balance of the cells.