A study has been made of the osmotic responses of the green intertidal alga, Ulva lactuca, under two fluctuating salinity regimes; sinusoidal and square-wave fluctuations between 30 and 100% sea water in a 12 h cycle. These regimes closely resemble the tidal fluctuation of salinity encountered by the alga in its natural estuarine habitat. Data on changes in the inorganic ions, potassium, sodium, chloride and sulphate; in the organic solute, dimethylsulphoniopropionate; in the total sugar levels and estimated osmotic and turgor pressures under the two salinity regimes are reported. Significant differences in the solute responses under these different conditions were detected. In general, better control of ion fluxes appeared to be exercised under the sinusoidal conditions which also buffered changes in dimethylsulphoniopropionate levels. Influxes of potassium were highly light-dependent. Chloride levels conspicuously failed to reach the steady-state levels in the 6-h-hyper-osmotic part of either the abrupt or gradual cycle. The possible significance of these data, which may better reflect osmotic changes in the natural environment, and some of the problems encountered, particularly in accounting for charge balance under some conditions, are discussed.