Hajibagheri, M. A., Gilmour, D. J., Collins, J. C. and Flowers, T. J. 1986. X-ray microanalysis and ultrastructural studies of cell compartments of Dunaliella parva. -J. exp. Bot. 37: 1725–1732. Ultrastructural studies of the unicellular green alga Dunaliella parva showed the presence of cytoplasmic vacuoles. X-ray microanalysis was performed on sections of cells which had been freeze substituted in acetone. It was found that the concentrations of both Na and Cl were much higher in the vacuoles than in the cytoplasm. When cells were grown in 0·4 kmol m−3 NaCl the Na and Cl concentrations in the vacuoles were 349 and 283 mol m −3 respectively, whilst cytoplasmic Na and Cl concentrations were 37 and 26 mol m−3. Corresponding values for cells grown in 1·5 kmol m−3 NaCl were 392 mol m−3 Na and 325 mol m−3 Cl in the vacuoles and 36 mol m−3 Na and 30 mol m−3 Cl in the cytoplasm. Immediately after exposure to an increase in external salinity Na and Q concentrations increased in both vacuoles and cytoplasm. The results are discussed with reference to compartmental models for the ionic relations of Dunaiiella.