A sublittoral population of Baltic Fucus vesiculosus L. and an intertidal population from the Irish Sea have been compared in several aspects of ecophysiology under a wide range of short-term (24 h) salinity treatments (1.5 to 102 ppt). Both populations exhibited net photosynthesis over the range of salinity and in general Atlantic plants were more productive. Optimum salinity for photosynthesis was different for Baltic (6 ppt) and Atlantic (12 to 34 ppt) populations. There were no differences in total tissue water between the populations, but Atlantic plants contained a greater proportion of intracellular water. Salinity had an effect on the accumulation of osmotically active compounds and populations differed significantly in this respect; Atlantic contained higher ion and mannitol levels than Baltic. Both populations discriminated in favour of K in preference to Na and it seemed that KC1 was the main inorganic solute responsible for osmotic balance. Based on 42K tracer experiments there was an unexchangeable pool of K in both populations. After short-term salinity exposure mannitol showed no clear increase in Baltic plants, however mannitol levels were higher after long-term exposure (1 1 wk, 1.5 to 45 ppt). In long-term experiments tissue ion contents were lower and differences in ionic relations were not so evident between the populations as compared to the short-term exposure data. Intraspecific differences in physiology of F vesiculosus suggest that ecotypic differentiation has occurred to some extent between marine and brackish water populations.