The osmoacclimation of Ectocarpus siliculosus isolates known to have different salt tolerances was investigated. Included were isolates originating from 5 different locations in the northern hemisphere, and sporophyte and gametophyte phases of different ploidy from two of the locations were compared. The effect of salinity treatment (8–64%0) on inorganic ions (K+, Na+, Mg2+, Cl−, SO2−4, phosphate) and the low molecular weight carbohydrate mannitol was measured, together with complimentary measurements of cell viability. Very different responses between isolates were obtained, both between isolates of different geographic origin and between sporophytes and gametophytes from the same parent material. A similarity in response between haploid and diploid gametophytes, and diploid and triploid sporophytes indicates that physiological differences between gametophyte and sporophyte generations are not necessarily based on ploidy changes alone. There were no identifiable differences in the responses of male and female gametophytes. K+ is the major osmolyte within the species, and differences in the regulation of K+ largely account for the observed variation in osmoacclimation, both between life history phases and between isolates from different localities. Isolates with broader salt tolerances had the higher concentrations of mannitol. There were differences between isolates in the amounts and regulation of Cl− and phosphate, the latter being present in unusually high concentrations. There were also isolate differences in the concentrations of Mg2+ and SO2−4, although these divalent ions were present only in low concentrations.