To understand the unique success of the marine seaweedFucus vesiculosus L. (PHaeophyceae) in the brackish Baltic Sea, the performance of gametes from Baltic [4.1–6.5‰S (Salinity)] and marine populations was studied. Sperm from BalticF. vesiculosus swam with a path velocity of c. 30–110 μm/s and could fertilize eggs in waters of salinities from 4 to 33‰S. In their natural water, Baltic sperm were not negatively phototactic, unlike marine sperm in seawater; this should decrease the sperm:egg concentration at the seafloor and reduce the likelihood of polyspermy. Marine (Iceland, Sweden) sperm in seawater had a path velocity of c. 80–100 μm/s, but performed poorly and could not fertilize eggs in natural or artificial Baltic water ≤6‰S; therefore, Baltic populations have adapted or acclimated to their brackish habitat. Baltic populations appear better adapted to their natural low salinities because, even after culturing Baltic and marine individuals in water from both the Baltic (6.5‰S) and the marine Skagerrak (21‰S), Baltic sperm were in both cases still able to swim and fertilize eggs at lower salinities (4‰S) than marine sperm; fertilization never occurred between marine gametes at 4–6‰S. However,F. vesiculosus acclimates to some salinities, since sperm from Baltic and marine males that had been cultured at 21‰S swam better (higher velocity, proportion that were motile and/or linearity) in marine salinities (21–33‰S) than when they were cultured at 6.5‰S. The effects of salinity on sperm motility and fertilization were osmolar rather than due to specific ionic requirements, over the tested range. The osmolalities (< c. 100 mmol/kg) at which fertilization success of Baltic gametes decreases nearly to zero correspond to the osmolality of Baltic water at the northernmost limit of distribution ofF. vesiculosus in the Baltic Sea. Therefore, the present range ofF. vesiculosus in the Baltic appears to correspond to the osmotic tolerance of the gametes. Very small natural or anthropogenic increases in ambient osmolality would be likely to cause a substantial expansion of this species into the inner Baltic.