AbstractFortyEctocarpus siliculosusisolates from a wide geographical range, including gametophyte and sporophyte plants, have all been acclimated to the same salinity for several years. Their salinity tolerances in respect of cell viability, photosynthesis and dark respiration were evaluated over the salinity range: 8 to 96 ‰. Significant differences in the physiological tolerances to salt stress compared with viability measurements were evident. Genotypic differences in salt tolerances between groupings of the isolates, and also differences in responses of gametophyte and sporophyte generations were found. However, diploid and haploid sporophyte material had similar tolerances. Triploid and tetraploid sporophytes did not have improved tolerances over those of diploid plants. Culture plants originating from low salinities in the Baltic Sea had broader tolerances than field material collected from Baltic waters of similar salinit